MutexAtomicFixnum and JavaAtomicFixnum are now classes

Updated `AtomicFixnum` to follow the recent pattern set by `FixedThreadPool` and `CachedThreadPool`. `MutexAtomicFixnum` and `JavaAtomicFixnum` are both now independent classes. Both will be defined when running under JRuby but only `MutexAtomicFixnum` will be defined under non-JRuby platforms. Each platform will then define `AtomicFixnum` as a subclass of the appropriate base class.