Created MRI and JRuby specific implementations of FixedThreadPool

* Added (untested) Concurrent::Processor_count function
* Memoized processor_count
* Created shared specs for fixed thread pools.
* Renamed FixedThreadPool to RubyFixedThreadPool
* FixedThreadPool now cast as platform-specific implementation.
* JavaFixedThreadPool now passing all specs.
* All specs passing for all fixed thread pool implementations.