[crac] RFR: Ensure empty Reference Handler and Cleaners queues [v2]

[Anton Kozlov]

> At the time of checkpoint, a set of References may need handling. This change ensures no References pending in ReferenceHandler and in Cleaners.
> 
> System.gc() is a best effort attempt to make GC to look for References. Default VM flags (-DisableExplicitGC, -ExplicitGCInvokesConcurrent) should not block the call, but additional investigation is needed to make sure GC found all references.

Anton Kozlov has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  Ensure empty Reference Handler and Cleaners queues

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Changes:
  - all: https://git.openjdk.java.net/crac/pull/13/files
  - new: https://git.openjdk.java.net/crac/pull/13/files/79096270..d26b2335

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=crac&pr=13&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=crac&pr=13&range=00-01

  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/crac/pull/13.diff
  Fetch: git fetch https://git.openjdk.java.net/crac pull/13/head:pull/13

PR: https://git.openjdk.java.net/crac/pull/13