Move sending HEARTBEAT code to PeerHandler; Use special request to quit processors.
Some fixes on code and some more failure test cases.
Pass just Request field to PreProcessor instead of whole request object.
Merge pull request #65 from EasonLiao/fix_preprocessor

Pass just Request field to PreProcessor instead of whole request object.
Merge pull request #61 from EasonLiao/failure_cases

Some more failure cases.