[crac] RFR: RCU Lock - RW lock with very lightweight read- and heavyweight write-locking

[Radim Vansa]

This implementation is suitable for uses where the write-locking happens very rarely (if at all), as in the case of CRaC checkpoint, and we don't want to slow down regular access to the protected resource.

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Commit messages:
 - RCU Lock - RW lock with very lightweight read- and heavyweight write-locking

Changes: https://git.openjdk.org/crac/pull/58/files
 Webrev: https://webrevs.openjdk.org/?repo=crac&pr=58&range=00
  Stats: 623 lines in 7 files changed: 622 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/crac/pull/58.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/58/head:pull/58

PR: https://git.openjdk.org/crac/pull/58