MB-11192: Snoozed backfills to have seperate queue

Seperate out snoozing backfills from active/running
backfills in the backfill manager.

Change-Id: If7e68515862f5e2089233969627f032c53939f97
Reviewed-on: http://review.couchbase.org/43274
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>