Before this patch, after `xslapos node stop wcfs`, `xslapos node start wcfs`
was failing in a sense that started service was turning into EXITED state
with something like the following in its log:

    wcfs: 2021/05/31 15:28:15 zodb: FIXME: open zeo://10.0.82.110:2100?storage=root: raw cache is not ready for invalidations -> NoCache forced
    Traceback (most recent call last):
      File "/srv/slapgrid/slappart34/srv/runner/software/44fe7dd3f13ecd100894c6368a35c055/bin/wcfs", line 290, in <module>
        sys.exit(wendelin.wcfs.main())
      File "<decorator-gen-7>", line 2, in main
      File "/srv/slapgrid/slappart34/srv/runner/software/44fe7dd3f13ecd100894c6368a35c055/develop-eggs/pygolang-0.0.8-py2.7-linux-x86_64.egg/golang/__init__.py", line 103, in _
        return f(*argv, **kw)
      File "/srv/slapgrid/slappart34/srv/runner/software/44fe7dd3f13ecd100894c6368a35c055/parts/wendelin.core/wcfs/__init__.py", line 409, in main
        serve(zurl, optv, exec_=True)
      File "/srv/slapgrid/slappart34/srv/runner/software/44fe7dd3f13ecd100894c6368a35c055/parts/wendelin.core/wcfs/__init__.py", line 365, in serve
        f = open(mntpt + "/.wcfs/zurl")
    IOError: [Errno 107] Transport endpoint is not connected: '/dev/shm/wcfs/c63ef16eb2e74dc62cabf8ffc11798fd42d0158a/.wcfs/zurl'

That's because when wcfs is killed (xslapos node stop), it is only filesystem
server process that is terminated, but the FUSE mount and connection remains
registered on the system, and any access to FUSE filesystem with exited/crashed
server returns that "Transport endpoint is not connected" error.

-> Fix that by detecting at wcfs start whether it was previous unclean wcfs
shutdown, and then pre-clean things as necessary.

This way multiple zopes, which all have their own separate $TMP and $TMPDIR,
can still access shared WCFS instance. And new place fits naturally into
general wendelin.core logic (see added comment).

It is not possible for WCFS to access data of in-RAM storage of another
process. But without explicit explanation the error message is confusing
- it was something like:

    NotImplementedError: don't know how to extract zurl from <ZODB.MappingStorage.MappingStorage object at 0x7f28f04cea10>

which suggests it was just not implemented.

I'm working on the fix. In the meantime don't let the failure misguide
potential users of WC2 draft.

treediff(ø, b) is TODO

We need to test this because the following case happens often in
practice: the tree did not exist when ΔBtail was started, later the tree
is created and ΔBtail is recomputed. Since we are testing tree transitions
that involves delete, test it uniformly for all cases.

Currently Update test, if Fatal("TODO") is removed fails with:

    --- FAIL: TestΔBTail/Update/T2,5/B1:a-B2:b,4:d-B8:h→ø (0.00s)
panic: ΔBtail update 03e036b7f59f86cc -> 03e036b7f5a28899: treediff 03e036b7f59f86cc..03e036b7f5a28899 000000000000000c: diffT 03e036b7f59f86cc:000000000000000c 03e036b7f5a28899:000000000000000c: BTrees.LOBTree.LOBTree(000000000000000c): activate: file:///tmp/δBTail851157667/1.fs: load 03e036b7f5a28899:000000000000000c: 000000000000000c: object was deleted @03e036b7f5a28899 [recovered]

I broke wcfs test while trying to implement rebuild.
Keep wcfs tests passing while rebuild is not finished yet.