For ZBigFile + WCFS client integration we'll need to open WCFS
connections that observer database at the same state as current ZODB
connection. Later that WCFS connection needs to adjust its on-WCFS view
in accordance to how ZODB connection adjusts its one.

Wczsync provides a function to do so: pywconnOf(zconn) will open WCFS
connection and maintain it in sync with ZODB connection zconn.

Some preliminary history:

8bf8f23b    X bigfile/_file_zodb: Fix logic around ZSync usage
571cb737    fixup! X bigfile/_file_zodb: Fix logic around ZSync usage
a9a82d5a    X bigfile/_file_zodb: Fix ZSync to close not only wconn, but also wconn.wc through which wconn was created
cf92937f    X wcfs: Move wconn<->zconn sync functionality into wcfs.client._wczsync
7203d7ab    X wcfs: Fix ZSync to close wconn on zdb.close, even if zconn stays alive