- it is more logical
- this way !wcfs tests don't try to excerceise wcfs functionality (and
  unexpectedly spawn wcfs server).

Since ZBigFile keeps references to fileh objects that are created
through it it forms a file <=> fileh cycle that is not collected without
cyclic GC:

https://lab.nexedi.com/nexedi/wendelin.core/blob/v0.13-52-ga702d41/bigfile/file_zodb.py#L497
https://lab.nexedi.com/nexedi/wendelin.core/blob/v0.13-52-ga702d41/bigfile/file_zodb.py#L566-571

We did not noticed this leak until now because it is small, but with
upcoming wendelin.core 2 it is important to release a fileh, because
there is WCFS connection associated with fileh, and if fileh is not
released, that connection also stays alive, keeping on-WCFS resources
still being used, and preventing WCFS from being unmounted cleanly.

-> Add cyclic GC support to PyBigFile / PyBigFileH

NOTE: we still don't allow PyVMA <=> PyBigFileH cycles to be collected,
because fileh_close called from fileh.__del__ asserts that there are no
live mappings left. See added comments for details. There is no
known practical need to use such cycles, so this should be ok.

See also other patches on cyclic GC topic:

- 450ad804 (bigarray: ArrayRef support for BigArray)  // adds cyclic GC support for PyVMA
- d97641d2 (bigfile/py: Properly untrack PyVMA from GC before dealloc)

/proposed-for-review-on nexedi/wendelin.core!12

The logic in pyvma_traverse and pyvma_clear needs to be synchronized
with PyVMA deallocation. In the next patche we'll be amending this
logic, and it will help a reader to keep all those functions together.

For the reference: PyVMA support for cyclic GC was introduced in
450ad804 (bigarray: ArrayRef support for BigArray). See also d97641d2
(bigfile/py: Properly untrack PyVMA from GC before dealloc).

/proposed-for-review-on nexedi/wendelin.core!12

Since ZBigFile keeps references to fileh objects that are created
through it it forms a file <=> fileh cycle that is not collected without
cyclic GC:

https://lab.nexedi.com/nexedi/wendelin.core/blob/v0.13-52-ga702d41/bigfile/file_zodb.py#L497
https://lab.nexedi.com/nexedi/wendelin.core/blob/v0.13-52-ga702d41/bigfile/file_zodb.py#L566-571

We did not noticed this leak until now because it is small, but with
upcoming wendelin.core 2 it is important to release a fileh, because
there is WCFS connection associated with fileh, and if fileh is not
released, that connection also stays alive, keeping on-WCFS resources
still being used, and preventing WCFS from being unmounted cleanly.

-> Add cyclic GC support to PyBigFile / PyBigFileH

NOTE: we still don't allow PyVMA <=> PyBigFileH cycles to be collected,
because fileh_close called from fileh.__del__ asserts that there are no
live mappings left. See added comments for details. There is no
known practical need to use such cycles, so this should be ok.

See also other patches on cyclic GC topic:

- 450ad804 (bigarray: ArrayRef support for BigArray)  // adds cyclic GC support for PyVMA
- d97641d2 (bigfile/py: Properly untrack PyVMA from GC before dealloc)

X bigfile/_file_zodb: Fix ZSync to close not only wconn, but also wconn.wc through which wconn was created

pywconnOf, before creating wconn, performs wc=wcfs.join(zurl) which
creates new filesystem-level connection to WCFS server. This wc is used
only to create wconn. So if we do not close wc, after releaseing wconn,
it will leak opened file descriptor, to e.g. .wcfs/zurl and prevent
tests from finishing cleanly.

Don't use regular mutex to protect _zsyncReg updates as this can
deadlock because one of _zsyncReg mutators (on_zconn_dealloc) is invoked
by automatic GC that can be triggered any time.

The logic inside ZSync was correct, but it was incorrect to attach zsync
to zconn to stay alive and react when that zconn is garbage collected:
zsync._on_zconn_dealloc was not called because zsync itself was garbage
collected too.

This fixes many failures where wconn and associated pinner was not
released even though ZODB DB was correctly closed.

Excercise the logic that keeps wconn <-> zconn in sync.

Else, when runing tests intree `import wcfs` and `import wendelin.wcfs`
will give two different modules, and inspecting e.g. wendelin.wcfs at
teardown will see fresh module state (_wcregistry) because it was wcfs
which was used.

Also just `import wcfs` will raise ImportError when run out of tree.

Previously if an assert or something failed in spawned thread, the main
thread was usually spinning indefinitely = tests hang. -> Switch all
threading places to use sync.WorkGroup and this way if a thread fails,
all other threads are canceled and the exception is reported back to
wg.wait in main thread.

Pygolang egg provides "golang" python package, not "pygolang".
Bug introduced in 5c8340d2 (*: Use defer for dbclose & friends).

- mention in comments that _ZBigFileH not only proxies changes from
  virtmem -> ZODB, but also the other way: virtmem <- ZODB.
- refresh comments, fix typo.

It is valid to compare a Page and a VMA only if they belong to the same
fileh.

-> into vma_page_infilerange().

We will soon need to use this functionality from several places.

Start preparing vma early, not after the call to mem_valloc.
This codeflow will be more convenient when we add mmap-through-wcfs codepath.

We are soon going to use this functionality from several places.

The place to perform the mmap is changed slightly because vma_mmap_page
deduces prot from page->state and for that we have to finish preparing
page->state first.

Freed Page structs must be in PAGE_EMPTY state.

The code sequence of

        list_del(&page->lru);
        bzero(page, sizeof(*page)); /* just in case */
        free(page);

was open-coded several times.