RFR: 8281615: Deadlock caused by jdwp agent

[Zhengyu Gu]

There are scenarios that JDWP agent can deadlock on `classTrackLock` monitor. Following is the scenario in bug report.

**Java Thread** 
-   loads a class and post `JVMTI_EVENT_CLASS_PREPARE` event
- JDWP event callback handler calls `classTrack_processUnloads()` to handle the event.
- `classTrack_processUnloads()` takes `classTrackLock` lock, then tries to allocate a new bag under the lock.
- bag allocation code calls` jvmtiAllocate()`, which may be blocked by ongoing safepoint due to state transition.

If the safepoint is GC safepoint (prior to JDK16) or `VM_JvmtiPostObjectFree`  safepoint (JDK16 or later)

**VM Thread**
- post `JVMTI_EVENT_OBJECT_FREE`
- JDWP event callback handler calls `cbTrackingObjectFree()` to handle the event
- `cbTrackingObjectFree()` tries to acquire `classTrackLock` lock, leads to deadlock

>From my research, there are three events that may be posted at safepoints, `JVMTI_EVENT_GARBAGE_COLLECTION_START`, `JVMTI_EVENT_GARBAGE_COLLECTION_FINISH` and  `JVMTI_EVENT_OBJECT_FREE`, but only  `JVMTI_EVENT_OBJECT_FREE` is relevant to JDWP agent.

The solution I purpose here, is simply move allocation/deallocation code outside of `classTrackLock` lock.


Test:
- [x] tier1 
- [x] vmTestbase_nsk_jdi
- [x] vmTestbase_nsk_jdwp
- [x] vmTestbase_nsk_jvmti

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Commit messages:
 - Update comments
 - v0

Changes: https://git.openjdk.java.net/jdk/pull/7461/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7461&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281615
  Stats: 25 lines in 1 file changed: 21 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7461.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7461/head:pull/7461

PR: https://git.openjdk.java.net/jdk/pull/7461