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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
https://lab.nexedi.com/kirr/wendelin.core/co...