(SERVER-150) flush jruby instances

I think of this as sort of a 'constructor' for the agent, where it gets a reference to the common things that it will need for all of its tasks, and hangs on to them indefinitely.  I think that having to grab a new reference to `shutdown-on-error` and pass it through the call stack to all of the eventual `send`s would be messier and more verbose than this.

To answer your question: at the moment, no, I don't really intend to do anything with the agent in the future that would involve 'statefulness'; I'm basically just using it as an observable scheduling mechanism.