Prelim RFR: JDK-8214097: Rework thread initialization and teardown logic

[David Holmes]

This is a preliminary RFR to get feedback. I won't be pushing until 13 
and the final RFR will have to sync with other changes that may impact 
what I'm doing here - e.g. Shenandoah GC.

@Markus: does this help with your JFR event hooks?

Bug: https://bugs.openjdk.java.net/browse/JDK-8214097
Webrev: http://cr.openjdk.java.net/~dholmes/8214097/webrev/

The primary change is that we introduce initialization and tear-down 
hooks for the different thread types so that we can consistently apply 
common actions - like adding/removing NJTs to the NJT::list - and ensure 
consistent lifecycle management. This is done by introducing virtual 
pre_run() and post_run() hooks so that we arrange the non-virtual 
Thread::call_run() so that it does:

void Thread::call_run() {

   // Perform common initialization actions

   register_thread_stack_with_NMT();
    ...

   // Perform <ChildClass> initialization actions
   this->pre_run();

   // Invoke <ChildClass>::run()
   this->run();

   // Perform common tear-down actions

   // Perform <ChildClass> tear-down actions
   this->post_run();

}

And establish some basic lifecycle rules, such as:
- no deletion of the thread is allowed prior to post_run()
- after post_run() 'this' should not be referenced
- Thread::current must be cleared before Thread deletion

I refactored a little bit of common setup by getting rid of 
NamedThread::initialize_named_thread(), and having NJT::pre_run() set 
the name.**

Added some comments about how the different thread entry points and 
hooks connect together.

Clarified some initialization related interactions between thread 
creation and BarrierSet creation (which Kim is trying to clean up under 
8215097).

Reverted the change in management.cpp put in under JDK-8212207 as it is 
no longer needed now that NJTs only get added to the list in pre_run() 
and so must be valid. And somewhat related I added an assert on Linux 
that osthread->pthread_id() has been set before the call to call_run().

Simplified gtest/threadHelper.inline.hpp as the hooks introduced are now 
supplied by Thread.

Thanks to Kim Barrett for the updated NJT list management code and 
discussion on different approaches.

Thanks,
David

** The role of NamedThread seems quite tenous now, but it's still being 
used to store "stuff" that seems unrelated to being a NamedThread. 
Anyway that would be a future cleanup RFE.