Implement a pause function for actors

[mwvaughn]

We cannot easily stop an actor (temporarily) from accepting new work. This can be inconvenient when trying to test parallel sets of linked actors. It would be handy to be able to pause/resume an actor without deleting it.

[mwvaughn]

Stop processing work => shut down all workers. Need to tag them as paused since the autoscaler will helpfully scale them back up!