nsqadmin: /empty_channel should redirect back to channel
Merge pull request #145 from mreiferson/empty_redirect_145

nsqadmin: empty channel should redirect back to channel
nsqd: empty should clear inflight and deferred

* refactor empty/flush code
Merge pull request #144 from mreiferson/empty_144

nsqd: empty queue does not empty deferred/inflight
nsqlookupd: introduce V2 protocol (framed)

* eliminate SendResponse() and ReadResponse() in favor of
  always using ReadUnpackedResponse()
fixes after rebase
nsqd: if we error sending heartbeat exit
nsqlookupd: a bit of V2 refactoring to support heartbeats
improved lookup_peer
cleanup
more refactoring