RFR: 8256304: should MonitorUsedDeflationThreshold be experimental or diagnostic

[Daniel D.Daugherty]

A pair of trivial fixes for a couple of ObjectMonitor cleanups.

Tested with Mach5 Tier[1-3].

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Commit messages:
 - 8256301: ObjectMonitor::is_busy() should return bool
 - 8256304: should MonitorUsedDeflationThreshold be experimental or diagnostic

Changes: https://git.openjdk.java.net/jdk/pull/4137/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4137&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256304
  Stats: 6 lines in 4 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4137.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4137/head:pull/4137

PR: https://git.openjdk.java.net/jdk/pull/4137