Use rocksdb::WriteBatch in RaftLog::TruncateSuffix

[KernelMaker]

Use rocksdb::WriteBatch to remove all useless log in one batch, it better than calling rocksdb::Delete one by one

https://github.com/Qihoo360/floyd/blob/ca64b0c310aa5ea756268908aa3a72b002e8b17c/floyd/src/raft_log.cc#L123-L130

[baotiao]

can you fix this issue? And can you suggest some other issue in how to use rocksdb in floyd @KernelMaker

[KernelMaker]

done