RFR: 8255119: Monitor::wait takes signed integer as timeout

[David Holmes]

Please review this simple cleanup of the JVM `Monitor`/`PlatformMonitor` code so that timeouts are expressed as unsigned rather than signed values i.e. `int64_t`/`jlong` -> `uint64_t`.

Testing:
 - tiers 1-3
 - all builds in Oracle CI in tiers 1-5, plus shenandoah GC on LInux x64

Thanks

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Commit messages:
 - 8255119: Monitor::wait takes signed integer as timeout

Changes: https://git.openjdk.org/jdk/pull/11898/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11898&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8255119
  Stats: 28 lines in 6 files changed: 6 ins; 6 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/11898.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11898/head:pull/11898

PR: https://git.openjdk.org/jdk/pull/11898