MB-11192: Wake up snoozed backfills when seqno available

Wake up the backfillManagerTask whenever a desired seqno
has been persisted.

Change-Id: I090a935557e47dbbfc961fc9aef61ae238d5de68
Reviewed-on: http://review.couchbase.org/43275
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>