X bigfile/_file_zodb: Fix ZSync to close not only wconn, but also wconn.wc through which wconn was created

pywconnOf, before creating wconn, performs wc=wcfs.join(zurl) which
creates new filesystem-level connection to WCFS server. This wc is used
only to create wconn. So if we do not close wc, after releaseing wconn,
it will leak opened file descriptor, to e.g. .wcfs/zurl and prevent
tests from finishing cleanly.