(SERVER-150) flush jruby instances

This strikes me as a somewhat odd usage of the agent's "state"/value because it never changes.  I wonder if this  might change in the future ... do you have thoughts in mind of how we might further leverage the statefulness of agents?  If not, this code would probably be a bit simpler if the service functions that called `send-*-pool!` simply passed in a reference to `shutdown-on-error`.