RFR: 8212107: VMThread issues and cleanup [v4]

[Robbin Ehn]

> We simplify the vmThread by removing the queue and refactor the the main loop.
> This solves the issues listed:
> - It can create an extra safepoint directly after a safepoint.
> - It's not safe for a non-JavaThread to add safepoint to queue while GC do oops do.
> - The exposure of the vm operation is dangerous if it's a handshake.
> - The code is a hornets nest with the repetition of checks and branches
> 
> Passes t1-8, and a benchmark run.
> 
> If you want a smaller diff the commits contains the incremental progress and each commit passed t1.

Robbin Ehn has updated the pull request incrementally with one additional commit since the last revision:

  Update after Shipilev

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/228/files
  - new: https://git.openjdk.java.net/jdk/pull/228/files/f590c6cb..3c0395bb

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=228&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=228&range=02-03

  Stats: 25 lines in 2 files changed: 7 ins; 4 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk/pull/228.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/228/head:pull/228

PR: https://git.openjdk.java.net/jdk/pull/228