how to:
1.speed up rocksdb backend by truncating tables
2.don't need to clear if the database is empty
3.don't need to clear variables if it's empty
4.don't need to commit hbase each len-prefix for delete index-label
potential problems:
1.mysql may block if create task vertex-label when truncating table
2.rocksdb may miss CF if create task vertex-label when truncating table
3.hbase may block if truncate hbase with version<2.0
improve #14
Change-Id: I4b2393aea8b0fc63c1886e984a576a1f5808b25c
Zhangmei Li