RFR: 8274054: Add custom enqueue calls during reference processing

[Thomas Schatzl]

Hi all,

  can I have reviews for this split-out of JDK-8271880/PR#5037 that only adds the interface and the default implementation to add custom "enqueue" calls to the VM?

For G1, the default (wrong) queue call is still used, that will be added with the other modifications to g1 later.

Testing: tier1-3

Thanks,
  Thomas

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Commit messages:
 - Some indentation fixes
 - Add custom enqueue calls

Changes: https://git.openjdk.java.net/jdk/pull/5603/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5603&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274054
  Stats: 84 lines in 9 files changed: 53 ins; 3 del; 28 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5603.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5603/head:pull/5603

PR: https://git.openjdk.java.net/jdk/pull/5603