(SERVER-150) Add service function for flushing JRuby pool

This commit adds a new service function to the JRubyPuppet service.
The new function can be used to request a flush of the JRuby pool.

The function is implemented by creating a new (offline) JRuby pool
and starting to populate it; as soon as the first instance is ready,
we can swap it into the service and remove the old pool, and then
continue initializing the remaining instances and cleaning up the
old pool in parallel.  It uses clojure agents to coordinate the
creation of the new pool and the cleanup of the old pool.