BlueStore: speedup the performance of multi-replication flow by

BlueStore: speedup the performance of multi-replication flow by
switching the callback order in bluestore.

In Bluestore ack callback and commit callback are queued one by one in
the function "BlueStore;:_txc_finish_kv". Therefore, just only one callback
is needed to call, in order to improve performance. We do this by switching
the callback order in bluestore, and the callback work can be done in
sub_op_modify_commit and doesn't need to do it again in sub_op_modify_applied.

Signed-off-by: Pan Liu <pan.liu@istuary.com>