(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.
For the sake of consistency, what about renaming this to `isStale`?  (Or, alternatively, using `expired` elsewhere in this file for `stale`.)