Consider using a listeners pool

[massenz]

Currently, even if running in coroutines and using channels to distribute the workload, the entire application is (essentially) single-threaded, as we only start one coroutine per task (listener, pub/sub, etc.)

We should explore the possibility of running the workers concurrently - however, this opens up the possibility of race conditions for events which, even if arriving in order from SQS, may become out-of-order (and cause unwanted issues, with transitions being deemed invalid, incorrectly).

It is non-obvious how to "single-thread" per FSM, while keeping independent event stream running concurrently.

[massenz]

See also #58