Added a new option, log_sync, if turned on (default), sync operations will be logged.
Added a new option, log_sync, if turned on (default), sync operations will be logged.

columbia/xtern,hemingcui/xtern_with_paxos,hemingcui/xtern_with_paxos,hemingcui/xtern_with_paxos,columbia/xtern,columbia/xtern,hemingcui/xtern_with_paxos,columbia/xtern,columbia/xtern,hemingcui/xtern_with_paxos,columbia/xtern