PYCBC-267: [twisted] Unset lcb_active _before_ the event fires

Otherwise we might not end up delivering events when the event is
re-activated from within the handler.

Change-Id: I8df3925c683de7d90192cd0c24c68d957915298a
Reviewed-on: http://review.couchbase.org/40939
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>