[RFA/JVMTI] Better env list locking

[Tom Tromey]

>>>>> "Keith" == Keith Seitz <keiths@redhat.com> writes:

Keith> A long time ago, I noted errors in the environment list locking in
Keith> jvmti.cc. Well, at long last, the time has come to fix it. With the
Keith> recent merge, we now have access to ReentrantReadWriteLock, which is
Keith> what is now used to lock the environment list.

Keith> Comments/questions/concerns?

Can post_event or any callback it calls possibly throw an exception?
If so that will leave the lock held.

If not this is ok.  Thanks.

Tom