[Service] Turns Service into an Actor and splits service into its own files

[allenwang28]

This diff makes a few changes:

• Changes Service into an actor, allowing it to run on its own process, allowing it to scale out better.
  • This turns _call, __initialize__, _call_all(enqueue on all healthy replicas) , start_session, get_metrics, get_metrics_summary, terminate_session, stop, into endpoints
• Introduces a ServiceInterface. This retains the existing session APIs, but now creates endpoints over the actor_def endpoints. It specifically introduces:
  • choose(): run this endpoint on a replica
  • call(): run this endpoint on all replicas
• Fixes a bug where sess_id is an arg. Turns it into a kwarg cc @joecummings
• Refactors service.py into
  • interface.py (including session and ServiceInterface),
  • metrics.py (including ServiceMetrics),
  • service.py (the base Service actor),
• adds new tests

[joecummings]

One general question, but overall looks good

[joecummings]

Why is this needed?

[allenwang28]

added a comment in the docstring, but a few reasons:

• primarily so you don't have to interact with the Service actor, which IMO can be annoying
• pairs the proc_mesh with the Service actor
• later we might want to pass a reference to the service to other actors, without moving where it's placed. This doubles as a handle so we can make calls to it

[allenwang28]

moved to ServiceInterface