prefetch container optimization

To reduce cold start overhead -- if we have a good reason to believe that an action will be invoked soon -- then send a message to /init a container. I think this could be done by adding "init" messages to the kafka protocol. When receiving an "init" message, the invoker would /init a container if none is present already.

The obvious use case is sequences -- when a sequence is run, "init" all the actions in the sequence immediately.