Provide filesystem view of in-ZODB ZBigFiles, but do not implement support for
invalidations nor isolation protocol yet. In particular, because ZODB
invalidations are not yet handled, the filesystem does not update its data in
accordance with ZODB updates, and instead provides stale data view that
corresponds to the state of ZODB at the time when wcfs was mounted.

The main parts of this patch are:

- wcfs/wcfs.go is filesystem implementation itself together with overview.
- wcfs/__init__.py is python wrapper to spawn and interoperate with that filesystem.
- wcfs/wcfs_test.py is tests.

Some preliminary history:

fe7efb94    X start of wcfs
878b2787    X draft loading
d58c71e8    X don't overalign end by 1 blksize if end is already aligned
29c9f13d    X readBlk: Fix thinko in already case
59552328    X wcfs: Care to disable OS polling on us
c00d94c7    X workaround lack of exception chaining on Python2 with xdefer
0398e23d    X bytearray turned out to be copying data
7a837040    X print wcfs.py py-level traceback on SIGBUS (e.g. wcfs.go aborting due to bug/panic)
661b871f    X make sure tests don't get stuck even if wcfs gets killed -9 ...
2c043d29    X More effort to unmount failed wcfs.go
1ccc4478    X Use `with gil` + regular py code instead of PyGILState_Ensure/PyGILState_Release/PyRun_SimpleString
5dc9c791    X wcfs: Kill xdefer
91e9eba8    X wcfs: test: Register tFile to tDB early
a7138fef    X wcfs: mkdir /tmp/wcfs with sticky bit
1eec76d0    X wcfs: try to set sticky for /tmp/wcfs even if the directory already exists
c2c35851    X wcfs: tests: Factor-out waiting for a general condition to become true into waitfor
78f36993    X wcfs: test: Fix thinko in getting /sys/fs/fuse/connection/<X> for wcfs
bc9eb16f    X wcfs: tests: Don't use testmntpt everywhere
6dec74e7    X wcfs: tests: Split tDB into -> tDB + tWCFS
3a6bd764    X wcfs: tests: Run `fusermount -u` the second time if we had to kill wcfs
112720f3    X wcfs: tests: Print which files are still opened on wcfs if `fusermount -u` fails
bb40185b    X wcfs: Take $WENDELIN_CORE_WCFS_OPTIONS into account not only from under join
03a9ef33    X wcfs: Remove credentials from zurl when computing wcfs mountpoint
68ee5bdc    X wcfs: lsof tweaks
21671879    X wcfs: Teach entrypoint frontend to handle subcommands: serve, status, stop
b0642b80    X wcfs: Switch mountpoints from /tmp/wcfs/* to /dev/shm/*
b0ca031f    X wcfs: Teach join/serve to start successfully even after unclean wcfs shutdown
5bfa8cf8    X wcfs: Add start to spawn a Server that can be later stopped  (draft)
5fcec261    X wcfs: Run fusermount and friends with /bin:/usr/bin always on path
669d7a20    fixup! X wcfs: Run fusermount and friends with /bin:/usr/bin always on path
6b22f8c4    X wcfs: Teach start to start successfully even after unclean wcfs shutdown
15389db0    X wcfs: Tune _fuse_unmount to include `fusermount -u` error message into raised exception
153c002a    X wcfs: _fuse_unmount: Try first `kill -TERM` before `kill -QUIT` wcfs
3244f3a6    X wcfs: lsof +D misbehaves - don't use it
a126e709    X wcfs: Put client log into its own logger
ac303d1e    X wcfs: tests: -v  ->  show only wcfs.py logs verbosely
d671a9e9    X wcfs: Give more time to stop wcfs server