Heartbeat calls from executor to agent are reported as errors

These HEARTBEAT calls and events were added in MESOS-7564. 

HEARTBEAT calls are generated by the executor library, which does not have access to the executor's Framework/Executor IDs. The library therefore uses some dummy values instead, because HEARTBEAT calls do not really require required fields. When the agent receives these dummy values, it returns a 400 Bad Request. It should return 202 Accepted instead.