• factor bits that constitute ZODB stack into component/ZODB, provide two flavours: ZODB4 and ZODB5;
• add ZODB4-wc2 flavour that should be used if wendelin.core 2 runs on ZODB4. See ZODB!1 (closed) for details.
• add support for testing that ZODB4-wc2 on Nexedi testing infrastructure.

Please see details in the individual commits.

/cc @nexedi