Fiber finalizer fix

ThreadFiber's finalizer was triggering the creation of a new ThreadFiber instance. So, for every ThreadFiber instance that got garbage collected another one was created.

This behavior can lead to the finalizer queue filling up as quickly as it's emptied under heavy memory pressure and the JVM grinding to a halt.

This fixes the issue with TorqueBox integration tests under JRuby 1.7.5 hanging when they didn't under 1.7.4.