ReplicatedPG: use default priority for Backfill messages

ReplicatedPG: use default priority for Backfill messages

Backfill messages modify the stats on the replica and therefore
must be sent with the same priority as sub_op_modify to ensure
ordering.  Using recovery_op_priority caused the following
sequence:

1) Primary(1) sends MOSDPGBackfill FINISH with updated stats (v1)
2) Primary(1) sends SubOp modify for new client op with stats (v2)
3) Replica(2) receives SubOp with stats (v2)
4) Replica(2) receives MOSDPGBackfill FINISH with stats (v1)
5) Replica(2) responds and Primary(1) resets pgtemp making
    Replica(2) Primary(2)
6) PG stats on Primary(2) several ops old.

Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>