This patch updates ZEO4-wc2 to fix a data corruption bug [1]. It also
adds a backport from ZEO5 to always use pickle protocol 3 on the wire.
This is needed in order to support compatibility with new version of
WCFS, that we want to update to in the next patch [2], [3].

[1] nexedi/ZEO@22b2c7a3
[2] kirr/neo!8
[3] kirr/neo@1bb0eb24

/reviewed-by @kirr @jerome
/reviewed-on nexedi/slapos!1837