RFR: 8212879: Make JVMTI TagMap table concurrent [v6]

[Coleen Phillimore]

On Mon, 9 Nov 2020 20:39:40 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> Thanks @sspitsyn .  I'm going to leave the gc_notification code because structurally the two sides of the if statement are different and it's not a long function.  Thank you for reviewing the change.
>
> This change also passes tier 7,8 testing.

does this work?
I've added two commits from @kimbarrett that defer the ObjectFree posting to the service thread or to a place where it could be removed before posting.
I also remerged and added the call JvmtiTagMap::set_needs_cleaning() to shenandoah which works after merging the latest code from shenandoah.
Testing tiers 1-6 currently.  jvmti/jdi tests pass with G1 and ZGC stress options, and JVMTI tests pass with shenandoah.
Thanks! Coleen

-------------

PR: https://git.openjdk.java.net/jdk/pull/967