Merge pull request #40 from EasonLiao/sync_new

Implement phase 2: state synchronization
QuorumZabTest: add 2 test cases.

1. Make sure the leader can start up by itself without any follower.
2. Test the case where the leader and the follower have the same
   acknowledged epoch and the leader has a higher zxid.
Merge pull request #52 from m1ch1/master

QuorumZabTest: add 2 test cases.
broadcasting