(FOR REVIEW) (SERVER-151) Flush JRuby Pool endpoint

This PR adds a `jruby-pool` endpoint to the HTTP Admin API. This endpoint, when given a DELETE request, will clear the JRuby Pool. 

This is based off of https://github.com/puppetlabs/puppet-server/pull/313, which hasn't yet been merged, which is why there's a bunch of commits from Chris in this PR and why this is marked as for review.

Once https://github.com/puppetlabs/puppet-server/pull/313 gets merged I'll cherry pick my work on top of that.