Merge pull request #59 from m1ch1/master

netty transport bug fixes and improvements.
Code refactory : change the logic to different processors and use short circuit transport for LeaderHandler.
Merge pull request #57 from EasonLiao/refactory

Code refactory : change the logic to different processors and use short ...
Move sending HEARTBEAT code to PeerHandler; Use special request to quit processors.
Merge pull request #60 from EasonLiao/refactory

use 'request of death' to shut down processors