RFR: JDK-8281460: Let ObjectMonitor have its own NMT category

[Thomas Stuefe]

It would be useful to distinguish ObjectMonitor allocations in NMT, as a simple way to both to see the needed footprint as well as the number of allocated monitors.

Note, this needs [JDK-8281450](https://bugs.openjdk.java.net/browse/JDK-8281450) as a prerequisite.

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Commit messages:
 - mtObjectMonitor

Changes: https://git.openjdk.java.net/jdk/pull/7378/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7378&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281460
  Stats: 2 lines in 2 files changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7378.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7378/head:pull/7378

PR: https://git.openjdk.java.net/jdk/pull/7378