RFR: 8373819: GenShen: Requested generation may be null [v3]

[William Kemper]

> This PR attempts to simplify the generational control thread by decoupling it somewhat from the heap/gc cancellation mechanism. This is meant to prevent the control thread from seeing inconsistencies between `shHeap::_cancelled_gc` and `shGenControlThread::_requested_gc_cause`.

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 13 commits:

 - Merge remote-tracking branch 'jdk/master' into fix-null-generation-crash
 - Fix typo in assertion message
 - Take regulator thread out of STS before requesting GC
   
   The request may block while it waits for control thread to stop old marking. If workers are already in the STS, and the regulator thread is still in the STS, but cannot yield, the safepoint will not run. Control, worker and regulator threads deadlock each other.
 - Add comments
 - Revert back to what should be on this branch
 - Merge remote-tracking branch 'jdk/master' into fix-null-generation-crash
 - Don't know how this file got deleted
 - Carry over gc cancellation to gc request
 - Do not let allocation failure requests be overwritten by other requests
 - Fix degen point handling
 - ... and 3 more: https://git.openjdk.org/jdk/compare/4458cab4...8f4f55db

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Changes: https://git.openjdk.org/jdk/pull/28932/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28932&range=02
  Stats: 95 lines in 4 files changed: 45 ins; 17 del; 33 mod
  Patch: https://git.openjdk.org/jdk/pull/28932.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28932/head:pull/28932

PR: https://git.openjdk.org/jdk/pull/28932