Use parent thread for Thread.current within a fiber - #5827

This is the start of experiments in making Thread.current, among
other things, pretend that a Fiber is equivalent to its master
thread, rather than being its own thread.

This work was triggered by me running the socketry/async specs and
seeing that there's some locking done across fibers that's
expected to treat them all as owning the lock. In this case, it's
an RSpec reentrant lock implementation that aggregates a Mutex and
the thread that acquired it and compares that thread with
Thread.current to avoid relocking.

For JRuby the following changes were needed to make the lock work:

* Thread.current must point at the "Fiber current thread" rather
than the "Fiber thread". We have attempted this in the past and
run afoul of system-level locking that saw through to the real
locks.
* Mutex#owned? had to do a similar check, aggregating a reference
  to the Fiber current thread that originally locked it and using
that for comparison with the current Fiber current thread.

With these changes the async specs run to completion with 16
errors that I have not looked into yet.