The use of this code path is very limited at the moment,
and this patch further simplifies the code to make it easier
to either deprecate or extend it in the future.
* Renamed service_status.py -> heartbeat_emitter.py.
* Merged HeartbeatEmitter and Heartbeat.
Change-Id: I68a4a49dd3e6bbc123c3ed5d3d3661aa08ad0bbb