* master: (27 commits)
  setup: Fix hooking of git_lsfiles in PEP517 mode
  lib/zodb: Add zstor_2zurl - way to convert a ZODB storage into URL to access it
  lib/zodb: Add patch to ZODB.Connection to support callback on connection DB view change
  lib/zodb: Add zconn_at draft (ZODB5 only)
  lib/zodb: Add zmajor - way to know under which ZODB 3, 4 or 5 we are running
  bigfile/zodb: Cosmetics
  bigfile/file.h: Cosmetics
  bigfile/virtmem: vma_page_addr: Kill wrong XXX
  bigfile/virtmem: Factor-out checking whether `page->fpgoffset` is in file-range covered by `vma`
  bigfile/virtmem: fileh_mmap: Refactor a bit
  bigfile/virtmem: vma_mmap_page: Don't skip mmap_page error silently
  bigfile/virtmem: Factor-out mmaping Page into VMA into vma_mmap_page
  bigfile/virtmem: page_del: Assert that Page is not on fileh->dirty list
  bigfile/virtmem: Factor-out Page struct freeing into page_del
  bigfile/zodb: Factor-out patch for ZODB.Connection.onOpenCallback into -> lib/zodb
  *: Kill unused imports
  bigfile/py: Move data structures to public .h file
  bigfile/py: Provide package-level documentation
  bigfile/py: Stop using Plan9 C extensions
  bigfile/zodb: Factor-out LivePersistent into -> lib/zodb
  ...

In PEP517 mode setup.py is sourced - not executed - and the build fails
with ImportError like this:

    Preparing wheel metadata ... error
    ERROR: Command errored out with exit status 1:
     command: /home/kirr/src/wendelin/venv/z-dev/bin/python2 /home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py prepare_metadata_for_build_wheel /tmp/tmp2F3aEs
         cwd: /home/kirr/src/wendelin/wendelin.core
    Complete output (53 lines):
    running dist_info
    creating /tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info
    writing requirements to /tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info/requires.txt
    writing /tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info/PKG-INFO
    writing top-level names to /tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info/top_level.txt
    writing dependency_links to /tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info/dependency_links.txt
    writing entry points to /tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info/entry_points.txt
    writing manifest file '/tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info/SOURCES.txt'
    package init file '__init__.py' not found (or not a regular file)
    Traceback (most recent call last):
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 257, in <module>
        main()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 240, in main
        json_out['return_val'] = hook(**hook_input['kwargs'])
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 110, in prepare_metadata_for_build_wheel
        return hook(metadata_directory, config_settings)
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/build_meta.py", line 155, in prepare_metadata_for_build_wheel
        self.run_setup()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/build_meta.py", line 234, in run_setup
        self).run_setup(setup_script=setup_script)
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/build_meta.py", line 141, in run_setup
        exec(compile(code, __file__, 'exec'), locals())
      File "setup.py", line 374, in <module>
        """.splitlines()]
      File "/home/kirr/src/tools/go/pygolang/golang/pyx/build.py", line 118, in setup
        setuptools_dso.setup(**kw)
      File "/home/kirr/src/tools/py/pypa/setuptools_dso/src/setuptools_dso/__init__.py", line 37, in setup
        _setup(**kws)
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/__init__.py", line 145, in setup
        return distutils.core.setup(**attrs)
      File "/usr/lib/python2.7/distutils/core.py", line 151, in setup
        dist.run_commands()
      File "/usr/lib/python2.7/distutils/dist.py", line 953, in run_commands
        self.run_command(cmd)
      File "/usr/lib/python2.7/distutils/dist.py", line 972, in run_command
        cmd_obj.run()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/command/dist_info.py", line 31, in run
        egg_info.run()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/command/egg_info.py", line 296, in run
        self.find_sources()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/command/egg_info.py", line 303, in find_sources
        mm.run()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/command/egg_info.py", line 534, in run
        self.add_defaults()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/command/egg_info.py", line 574, in add_defaults
        rcfiles = list(walk_revctrl())
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/command/sdist.py", line 20, in walk_revctrl
        for item in ep.load()(dirname):
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2434, in load
        return self.resolve()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2444, in resolve
        raise ImportError(str(exc))
    ImportError: 'module' object has no attribute 'git_lsfiles'

See comments added to register_as_entrypoint for explanation of what
happens.

Wendelin.core will soon switch to PEP517 mode (by adding pyproject.toml)
to build-require Cython, Pygolang and friends.

Wendelin.core 2 will need to spawn WCFS filesystem server that accesses
the same ZODB database as the program that spawns it. The database
argument passed to WCFS is passed in the form of URL[1,2].  Even though
zodburi provides way to convert an URL into ZODB storage instance, there
is currently no way for reverse operation - to convert ZODB storage
instance into URL to access it(*). So we have to build it by our own.

Provide zstor_2zurl stub that currently works for FileStorage only.
ZEO and NEO support is TODO.

In the future we might want to move this functionality into
zodbtools/py.

[1] https://lab.nexedi.com/nexedi/zodbtools/blob/a2e4dd23/zodbtools/help.py#L27-53
[2] https://lab.nexedi.com/kirr/neo/blob/3d909114/go/zodb/zodbtools/help.go#L25-51

(*) contrary to ZODB/go where this functionality is provided out of the box:
    https://godoc.org/lab.nexedi.com/kirr/neo/go/zodb#IStorage

Wendelin.core 2 will need to hook into when client ZODB.Connection
changes its database view and readjust WCFS-level client connection
accordingly.

ZODB.Connection can change its view on either connection reopen, or even
without reopen on start of new transaction.

This patch implements ZODB.Connection.onResyncCallback for ZODB5 only.

ZODB4 and ZODB3 support is TODO.