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:

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