By using WCFS as mmap-overlay for base data(*). WCFS-mode is still opt-in
with default remaining to use old full user-space virtual memory manager
mode as initially introduced in 2015.

Wendelin.core should be draftly usable in WCFS mode now.

This patch is organized as follows:

- file_zodb.cpp provides mmap-overlay operations for WCFS implemented via
  WCFS client library.
- file_zodb.py is adjusted accordingly to use WCFS if requested.
  Low-level things specific to gluing to file_zodb.cpp are moved to _file_zodb.pyx.
- the rest of the changes are drive-by by main ones.

(*) see the following patches for what is mmap-overlay:

- fae045cc  (bigfile/virtmem: Introduce "mmap overlay" mode)
- 23362204  (bigfile/py: Allow PyBigFile backend to expose "mmap overlay" functionality)

Some preliminary history:

01916f09    X Draft demo that reading data through wcfs works
fd58082a    X Fix build on old GCC
f622e751    X tests: Stop wcfs spawned during tests
f118617b    X tests: Don't try to stop wcfs that is already exited