10 seconds is apparently not enough on a busy machine: with it I'm seeing
several failures where glog is stuck in fsync flushing its buffers, for
example:

https://nexedijs.erp5.net/#/test_result_module/20210817-E9535884/25
https://nexedijs.erp5.net/#/test_result_module/20210817-130D3B7A2/23
https://nexedijs.erp5.net/#/test_result_module/20210817-130D3B7A2/21
...

let's try to double the timeout.

X wcfs/xbtree: Fix update not to add duplicate extra point if rebuild  - called by Update - already added it

It was failing e.g. as

=== RUN   TestΔBTail/rebuild/T4/T1,3-T/T-T-T-T/B0:b-B1:c,2:j-T-B4:d/B3:h→T/T2,3/T-T-T/B1:d-B2:c-B3:i/_T{3};R/_→T2/B1:g-B2:c,3:i
    δbtail_test.go:917: after Update(@at1→@at2): vδT:
        have:
        	@at1: map[0:{b ø} 1:{c d} 2:{j c} 3:{h i} 4:{d ø}]
        	@at2: map[1:{d g}]
        vδb/root:
        	@at1
        	@at2
        	@at2	<-- HERE

https://nexedijs.erp5.net/#/test_result_module/20210816-95C3A93E/5

xbtree tests, in normal mode, run ~ 130s on my laptop. On a testnode
however they run ~500s and sometimes more than 10 minutes, probably
depending on surrounding load.

-> increase default `go test` timeout to avoid sporadic "test timed out"
failures.

This is what we are interested in the first place.
For full details -vv will show *.py logs verbosely.

This makes sure those programs are always built afresh instead being
stuck at outdated build. This is needed because corresponding test .c
file includes many other .c files and we don't implement dependency
tracking.

There is no TransactionMetaData on ZODB4.

See added comments to wcfs_test.py for details on how that can happen.
Fixes test segmentation faults like

    $ WENDELIN_CORE_TEST_DB="<zeo>" python -m pytest -vs -k test_wcfs_watch_vs_access

    wcfs_test.py::test_wcfs_watch_vs_access
    ------------------------------- live log setup --------------------------------
    INFO     ZEO.ClientStorage:ClientStorage.py:263 ('localhost', 20106) ClientStorage (pid=36942) created RW/normal for storage: '1'
    INFO     ZEO.cache:cache.py:217 created temporary cache file '<fdopen>'
    INFO     ZEO.ClientStorage:ClientStorage.py:574 ('localhost', 20106) Testing connection <ManagedClientConnection ('127.0.0.1', 20106)>
    INFO     ZEO.zrpc.Connection('C'):connection.py:365 (127.0.0.1:20106) received handshake 'Z4'
    INFO     ZEO.ClientStorage:ClientStorage.py:580 ('localhost', 20106) Server authentication protocol None
    INFO     ZEO.ClientStorage:ClientStorage.py:640 ('localhost', 20106) Connected to storage: ('localhost', 20106)
    INFO     ZEO.ClientStorage:ClientStorage.py:1326 ('localhost', 20106) No verification necessary -- empty cache
    INFO     ZEO.ClientStorage:ClientStorage.py:728 ('localhost', 20106) Disconnected from storage: "('localhost', 20106)"
    -------------------------------- live log call --------------------------------
    INFO     ZEO.ClientStorage:ClientStorage.py:263 ('localhost', 20106) ClientStorage (pid=36942) created RW/normal for storage: '1'
    INFO     ZEO.cache:cache.py:217 created temporary cache file '<fdopen>'
    INFO     ZEO.ClientStorage:ClientStorage.py:574 ('localhost', 20106) Testing connection <ManagedClientConnection ('127.0.0.1', 20106)>
    INFO     ZEO.zrpc.Connection('C'):connection.py:365 (127.0.0.1:20106) received handshake 'Z4'
    INFO     ZEO.ClientStorage:ClientStorage.py:580 ('localhost', 20106) Server authentication protocol None
    INFO     ZEO.ClientStorage:ClientStorage.py:640 ('localhost', 20106) Connected to storage: ('localhost', 20106)
    INFO     ZEO.ClientStorage:ClientStorage.py:1326 ('localhost', 20106) No verification necessary -- empty cache
    INFO     root:__init__.py:294 wcfs: starting for zeo://localhost:20106 ...
    wcfs: 2021/08/13 02:27:40 zodb: FIXME: open zeo://localhost:20106: raw cache is not ready for invalidations -> NoCache forced
    INFO     root:__init__.py:335 wcfs: started pid37431 @ /dev/shm/wcfs/e7630c831aeed36692d06459de5a25a745eb9d76

    M: commit -> @at0 (03e2107fabf002ee)

    M: commit -> @at1 (03e2107fac097466)
    M:      f<0000000000000002>     [2]

    M: commit -> @at2 (03e2107fac3df2aa)
    M:      f<0000000000000002>     [2, 3, 5]

    M: commit -> @at3 (03e2107fac5ef011)
    M:      f<0000000000000002>     [2, 5]

    C: setup watch f<0000000000000002> @at3 (03e2107fac5ef011)
    #  pinok: {}

    C: setup watch f<0000000000000002> @at3 (03e2107fac5ef011)
    #  pinok: {}

    C: setup watch f<0000000000000002> @at2 (03e2107fac3df2aa)
    #  pinok: {2: @at2 (03e2107fac3df2aa)}

    M: commit -> @at4 (03e2107face33c77)
    M:      f<0000000000000002>     [2, 5, 6]

    >>> Change history by file:

    f<0000000000000002>:
                                    0 1 2 3 4 5 6 7
                                    a b c d e f g h
            @at0 (03e2107fabf002ee)
            @at1 (03e2107fac097466)     2
            @at2 (03e2107fac3df2aa)     2 3   5
            @at3 (03e2107fac5ef011)     2     5
            @at4 (03e2107face33c77)     2     5 6

    INFO     ZEO.ClientStorage:ClientStorage.py:728 ('localhost', 20106) Disconnected from storage: "('localhost', 20106)"
    INFO     root:__init__.py:401 wcfs: unmount/stop wcfs pid37431 @ /dev/shm/wcfs/e7630c831aeed36692d06459de5a25a745eb9d76
    WARNING  root:__init__.py:548 fuse_unmount /dev/shm/wcfs/e7630c831aeed36692d06459de5a25a745eb9d76: failed: fusermount: failed to unmount /dev/shm/wcfs/e7630c831aeed36692d06459de5a25a745eb9d76: Device or resource busy
    WARNING  root:__init__.py:533 # lsof /dev/shm/wcfs/e7630c831aeed36692d06459de5a25a745eb9d76
    WARNING  root:__init__.py:541
    WARNING  root:__init__.py:543 (lsof failed)
    WARNING  root:__init__.py:461 -> kill -TERM wcfs.go ...
    WARNING  root:__init__.py:464 -> abort FUSE connection ...
    Segmentation fault: read @00007f6e36bfe000
    /srv/slapgrid/slappart91/srv/runner/software/3335682bae677c2d474f9244e578f64b/parts/wendelin.core/wcfs/client/./../../bigfile/liblibvirtmem.so(dump_traceback+0x1b)[0x7f6f80844e4b]
    /srv/slapgrid/slappart91/srv/runner/software/3335682bae677c2d474f9244e578f64b/parts/wendelin.core/wcfs/client/./../../bigfile/liblibvirtmem.so(+0x3956)[0x7f6f80841956]
    /lib/x86_64-linux-gnu/libpthread.so.0(+0x12730)[0x7f6f83117730]
    /srv/slapgrid/slappart91/srv/runner/software/3335682bae677c2d474f9244e578f64b/parts/wendelin.core/wcfs/internal/wcfs_test.so(+0x10860)[0x7f6e3e2eb860]
    /srv/slapgrid/slappart91/srv//runner//shared/python2.7/93d57ff089fd75f374514794469a0538/bin/python2.7(PyEval_EvalFrameEx+0x7b5)[0x4d2dc5]
    /srv/slapgrid/slappart91/srv//runner//shared/python2.7/93d57ff089fd75f374514794469a0538/bin/python2.7(PyEval_EvalCodeEx+0x2cc)[0x4d1abc]
    /srv/slapgrid/slappart91/srv//runner//shared/python2.7/93d57ff089fd75f374514794469a0538/bin/python2.7[0x51b92e]
    /srv/slapgrid/slappart91/srv/runner/software/3335682bae677c2d474f9244e578f64b/develop-eggs/pygolang-0.0.8-py2.7-linux-x86_64.egg/golang/_golang.so(+0xc8b0)[0x7f6f8182b8b0]
    /srv/slapgrid/slappart91/srv/runner/software/3335682bae677c2d474f9244e578f64b/develop-eggs/pygolang-0.0.8-py2.7-linux-x86_64.egg/golang/_golang.so(+0x14ab4)[0x7f6f81833ab4]
    /srv/slapgrid/slappart91/srv//runner//shared/python2.7/93d57ff089fd75f374514794469a0538/bin/python2.7[0x54bbb4]
    /lib/x86_64-linux-gnu/libpthread.so.0(+0x7fa3)[0x7f6f8310cfa3]
    /lib/x86_64-linux-gnu/libc.so.6(clone+0x3f)[0x7f6f82eae4cf]
    Segmentation fault (core dumped)

Which looks under gdb as

    #0  on_pagefault (sig=<optimized out>, si=0x7f6dde7fb570, _uc=<optimized out>) at bigfile/pagefault.c:171
    #1  <signal handler called>
    #2  __pyx_pf_8wendelin_4wcfs_8internal_9wcfs_test_read_nogil (__pyx_self=<optimized out>, __pyx_v_mem=...) at wcfs/internal/wcfs_test.cpp:3103
    #3  __pyx_pw_8wendelin_4wcfs_8internal_9wcfs_test_1read_nogil (__pyx_self=<optimized out>, __pyx_arg_mem=<optimized out>) at wcfs/internal/wcfs_test.cpp:3029
    #4  0x00000000004d2dc5 in call_function (oparg=<optimized out>, pp_stack=0x7f6dde7fbc88) at Python/ceval.c:4364
    #5  PyEval_EvalFrameEx (f=<optimized out>, throwflag=<optimized out>) at Python/ceval.c:3013
    #6  0x00000000004d1abc in PyEval_EvalCodeEx (co=0x7f6f8094cbb0, globals=<optimized out>, locals=locals@entry=0x0, args=args@entry=0x7f6f82d72068, argcount=<optimized out>, kws=kws@entry=0x7f6f82d72068, kwcount=0, defs=0x0, defcount=0,
        closure=0x7f6e3c6e7110) at Python/ceval.c:3608
    #7  0x000000000051b92e in function_call (func=0x7f6e3c711150, arg=0x7f6f82d72050, kw=0x7f6e3c710b90) at Objects/funcobject.c:523
    #8  0x00007f6f8182b8b0 in __Pyx_PyObject_Call (func=0x7f6e3c711150, arg=<optimized out>, kw=<optimized out>) at golang/_golang.cpp:15660
    #9  0x00007f6f81833ab4 in __pyx_f_6golang_7_golang___goviac (__pyx_v_arg=0x7f6e3c70f5f0) at golang/_golang.cpp:3466
    #10 __pyx_f_6golang_7_golang__goviac (__pyx_v_arg=__pyx_v_arg@entry=0x7f6e3c70f5f0) at golang/_golang.cpp:3350
    #11 0x000000000054bbb4 in pythread_wrapper (arg=<optimized out>) at Python/thread_pthread.h:178
    #12 0x00007f6f8310cfa3 in start_thread () from /lib/x86_64-linux-gnu/libpthread.so.0
    #13 0x00007f6f82eae4cf in clone () from /lib/x86_64-linux-gnu/libc.so.6

This reverts commit 30740602.

libbracktrace is not always automatically installed.

Do what we can do without gdb and then tail to regular segmentation
fault. With core gdb can still be useed, but if we already get in the
log traceback of the crash automatically.

t(TestΔBTailRandom) ~320s -> ~100s.

* kirr/t:
  X Fix mlock2 build on Debian 8

* t2:
  X ΔFtail.SliceByFileRev: Fix untracked entries to be present uniformly in result
  .
  .
  .
  .
  .
  X test that shows problem of SliceByRootRev where untracked blocks are not added uniformly into whole history
  .
  .
  .
  .
  .
  .
  .
  .
  X Size no longer tracks [0,∞) since we start tracking when zfile is non-empty
  X ΔFtail: `go test -failfast -short -v -run Random -randseed=1626793016249041295` discovered problems