(SERVER-150) flush jruby instances

What about updating this test to simply drain the pool a second time here, which would ensure that the poison pill was acquired at some point and the retry occurred?  If that change was made, I'd be okay with removing the second test case below - I don't love its use of mocking, and I think that an updated version of this test might be sufficient coverage.

And I betcha @camlow325 would like to chime-in on this.