Even if ZODB DB is closed and releases zconn from its .pool, zconn can
still stay alive being referenced from any other live python object -
e.g. from a frame, a traceback etc. This situation in particular happens
under ERP5's runUnitTest.

As the result, if we don't hook into DB.close and close wconn only on
zconn GC, files opened on WCFS filesystem stay opened, and WCFS server
cannot be cleanly unmounted when test run completes.

-> Fix it.

P.S. ZODB@bbd03b3a is good, but does
not really solve this problem because, once again, zconn stays
referenced from objects besides DB.