Refactory

[EasonLiao]

I still need to add some comments to code and do some minor changes.

@m1ch1 if you have time, you could also take a look how it looks like.

[ghost]

It looks like the build failure is due to javadoc errors.

[EasonLiao]

@m1ch1
New changes :

• Removed Zab.
• Removed ServerState, use PersistentState for election.

[EasonLiao]

Fixed what mentioned in code review except :

• SyncPeerTask
• Batch send request before broadcasting phase.
• Also batch leave request before broadcasting phase.

I'll merge the code, for these I've not fixed, I opened issues for them.