RFR: 8366865: G1: Allocation GC Pauses Triggered after JVM has started shutdown

[Ivan Walulya]

Please review this patch to skip VM_GC_Collect_Operations  if initiated after the VM shutdown process has begun. We add a _is_shutting_down flag to CollectedHeap, which is set while holding the Heap_lock. This ensures mutual exclusion with VM_GC_Collect_Operations, which also require the Heap_lock.

Skipping VM_GC_Collect_Operation would otherwise cause allocation requests to fail (resulting in OutOfMemoryError) if requesting daemon threads were allowed to continue, we instead block these threads on a monitor. They remain stalled until they are terminated as part of the VM shutdown sequence.

Testing: Tier 1-7

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Commit messages:
 - Merge remote-tracking branch 'upstream/master' into shutting_down_gcs
 - space
 - remove debug logs
 - remove debug logs
 - log_cpu_time
 - init

Changes: https://git.openjdk.org/jdk/pull/27190/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27190&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366865
  Stats: 164 lines in 15 files changed: 96 ins; 62 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/27190.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27190/head:pull/27190

PR: https://git.openjdk.org/jdk/pull/27190