Merge pull request #23 from changbl/working

Improve ZooKeeper-relevant code
Serialization and deserialization of data on znode:

Add two functions tuple_to_zk_data and zk_data_to_tuple,
for transforming format of data between tuple and string.
Merge pull request #24 from powerist/working

Serialization and deserialization of data on znode:
Scale out flow priorities

So that more priorities can be accommodated in the future