slapos-master: Switch to ZODB4-wc2
SlapOS Master is going to use Wendelin.core 2.
Wendelin.core 2 works with either ZODB4-wc2, or ZODB5, but not with ZODB4(*).
ERP5 does not currently work with ZODB5.
-> this leaves ZODB4-wc2 as the only choice.
(*) see https://lab.nexedi.com/nexedi/ZODB/commit/8e7eab33?expanded=1 for details.