[crac] RFR: Synchronize concurrent clean() in PhantomCleanableRef

[Radim Vansa]

Fixes failures in RefQueueTest and JarFileFactoryCacheTest.

-------------

Commit messages:
 - Synchronize concurrent clean() in PhantomCleanableRef

Changes: https://git.openjdk.org/crac/pull/70/files
 Webrev: https://webrevs.openjdk.org/?repo=crac&pr=70&range=00
  Stats: 21 lines in 3 files changed: 16 ins; 4 del; 1 mod
  Patch: https://git.openjdk.org/crac/pull/70.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/70/head:pull/70

PR: https://git.openjdk.org/crac/pull/70