RFR: 8269466: Factor out the common code for initializing and starting internal VM JavaThreads

[David Holmes]

Please see the JBS issue for more details, but basically we have 8 different kinds of internal VM JavaThreads (grouping the three types of CompilerThread together) that all basically duplicated the logic for initializing (preparing is the term we use for user-defined JavaThreads) and starting the new thread. This common code can be factored out into static helpers in JavaThread.

This change does not look at the way the java.lang.Thread instance is created - that will be handled by a separate RFE.

The semantics of the changes are not identical, but I don't believe there is any observable change in behaviour. The scope of holding the Threads_lock has been reduced, and we now create the JavaThread instances ("new XXXThread(...)") outside of the lock. As far as I can see nothing in the construction process needs to happen under the Threads_lock.

A few of the threads use a static `_instance` field to hold a reference to the create JavaThread. This proved very difficult to handle, as logically the field would need to be updated in the middle of the new factored-out method: after setting all the fields but before releasing the newly started thread. I eventually realized that in all but one case those `_instance` fields are never used and so could be deleted. The one case remaining does not need to be set as I just described, but can be set after the thread has started, as the new thread does not examine it (arguably its existence is unnecessary).

The trickiest changes related to the CompilerThreads, so they need particular scrutiny.

Testing: tiers 1-3

Thanks,
David

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Commit messages:
 - Removed leftover experimental code
 - Remove unnedded include
 - Fixed old comment
 - Set native priority before starting the CompilerThread.
 - 8269466: Factor out the common code for initializing and starting internal VM JavaThreads
 - interim 2 - template method doesn't work
 - Interim

Changes: https://git.openjdk.java.net/jdk/pull/4629/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4629&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269466
  Stats: 278 lines in 13 files changed: 63 ins; 140 del; 75 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4629.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4629/head:pull/4629

PR: https://git.openjdk.java.net/jdk/pull/4629