RFR: CODETOOLS-7902918: jcstress: Replace the relevant uses of HashMultiset with Counter

[Aleksey Shipilev]

High-performance parts of jcstress code use open-address Counter to store state data. There is no reason why we cannot use Counter in the TestResults themselves.

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Commit messages:
 - Replace HashMultiset with Counter
 - Merge branch 'master' into multiset-to-counter
 - Merge branch 'master' into multiset-to-counter
 - Custom serializer for Counter<R>
 - Replace HashMultiset with Counter

Changes: https://git.openjdk.java.net/jcstress/pull/46/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jcstress&pr=46&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7902918
  Stats: 116 lines in 4 files changed: 79 ins; 20 del; 17 mod
  Patch: https://git.openjdk.java.net/jcstress/pull/46.diff
  Fetch: git fetch https://git.openjdk.java.net/jcstress pull/46/head:pull/46

PR: https://git.openjdk.java.net/jcstress/pull/46