New consumer HeartbeatRequestManager should rediscover disconnected coordinator

The HeartbeatRequestManager attempts to send heartbeat requests to the coordinator. In the case that it gets a DisconnectException, it simply retries the request with backoff (here), but it should rediscover the coordinator first (by calling "markCoordinatorUnknown") to ensure it sends the HBs to the right coordinator. 

Note that this disconnection case are properly handled in the CommitRequestManager (here) but are missing in the HeartbeatRequestManager.