Clarified workflow of client interface. 

Some clarifications of how Zab interacts with client based on Friday's discussion.
- Constructs QuorumZab for recovery ```  QuorumZab(Properties prop, StateMachine st)```. Constructs QuorumZab for joining ``` QuorumZab(Properties prop, StateMachine st, String peer)```.
- clusterChange callback will be invoked right before the "broadcasting" callback(```leading/following```). For join case, the first clusterChange callback for joiner is guaranteed to contain the joiner in the configuration (Means the COP is the last message for synchronization).