In 3bd82127 (lib/zodb: Add zconn_at draft (ZODB5 only)) we added
zconn_at function to find out as of which state a ZODB connection is
viewing the database. That was ZODB5-only however.

Let's add support for ZODB4 now - by requiring ZODB4-wc2 - a version of
ZODB4 with MVCC backported from ZODB5: nexedi/ZODB!1

This makes wendelin.core to work on either ZODB5 or ZODB4-wc2, but not
plain ZODB4. However as zconn_at will be used only for WCFS-integration,
non-wcfs mode will continue to work on all ZODB5, ZODB4-wc2 and plain
ZODB4.

ZBigFile + WCFS client integration will use zconn_at to open WCFS
connection that corresponds to ZODB connection.

Preliminary history:

1c3b7750    X zconn_at for ZODB4