RFR: 8373819: GenShen: Requested generation may be null

[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`.

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

Commit messages:
 - 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
 - Try to simplify control thread protocol
 - Remove instrumentation, add assertion and try a fix
 - Instrumentation and fix

Changes: https://git.openjdk.org/jdk/pull/28932/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28932&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373819
  Stats: 86 lines in 2 files changed: 38 ins; 17 del; 31 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