(SERVER-112) Add support for flushing environment cache

This commit introduces a new class that we can plug in to the
recently-added Puppet extension point for controlling the
eviction of entries from the directory environment cache.

It also introduces all of the corresponding plumbing that
we need on the clojure side in order to make it work.  This
includes some additional state that we need to track on each
JRuby instance to keep track of the staleness of individual
environments.

Lastly, it introduces some integration tests that actually
set up the puppet code on disk and make catalog requests
against the server to validate the caching behavior.
This test is pretty sweet.  However, it is dependent upon the implementation of the JRuby pool, and that makes me a bit uneasy.  Consider if we swapped-out the underlying data structure with one that randomly returned a JRuby instance, or returned them in initialization-order ... either of those would break this test.