Closes https://github.com/coder/scaletest/issues/151
Closes GRU-71
Use the existing MsgQueue from the original PGPubsub instead of the 2-channel solution originally built here.
Renames `natsSub` to `groupSub`, since conceptually, a "NATS Subscription" already refers to the underlying subscription on the NATS server.
This PR also simplifies the closing of the PubSub to just close each `localSub`. When the last `localSub` for an event is closed, it unsubscribes and remove the `groupSub`. This ensures we go through the same code paths closing normally and at end of day.
