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

[Kim Barrett]

> On Dec 10, 2018, at 2:07 AM, David Holmes <david.holmes at oracle.com> wrote:
> 
> 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/

Other than the bootstrapping code that should be simplified / removed in response to JDK-8215097,
this looks good.  I think it will address the problems I ran into with GC-related hook changes I want
to make as part of JDK-8209974.