Posting to a shutdown thread pool - JRuby consistency and better naming

As discussed in #199, this gives JRuby the same behaviour and renames the code to "fallback_X" rather than "overflow_X".

"rake spec" passes on JRuby 1.7.6 (which I think runs both the pure-Ruby and the Java-backed code?), and I've moved the relevant tests to be shared between all thread pool executors.