Run InterGeneratorRouter inside a monarch actor

[pzhan9]

InterGeneratorRouter has mutable states such as generator load and lifecycle state. Its current implementation is not safe to share across threads or processes. This blocks it from being used concurrently from multiple processes.

To solve this problem, this PR add a InterGeneratorRouterActor, which is a Monarch Actor wrapper. This actor is spawned on its dedicate process as a singleton Monarch actor mesh. In this way, the router can be accessed concurrently through this actor.

Currently the actor is only used by controller. This PR is mainly to set up the stage so we later can:

1. Put Rollouters on a process pool, where each process needs its own router access.
2. Put router behind a HTTP endpoint, where the HTTP server is in its own process, or maybe even a different host.