generalize warmup port usage as 'routing' port

[inexplicable]

on 'cluster3' branch, we already used 'warmUpPort' to assign each worker a different port for the purpose of a guaranteed warmup, there're couple of more use cases which we think require this pattern to be generalized, after all, a dedicated port is for the sake of routing traffic to a specific worker (as it alone listens on that port)

some sample use cases are:

1. testing a specific worker
2. using server-sent event for logging, which requires a url that would route to a deterministic worker process