RFR: JDK-8281460: Let ObjectMonitor have its own NMT category [v2]

[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.

Thomas Stuefe has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge
 - rename category as Dan suggested
 - mtObjectMonitor

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7378/files
  - new: https://git.openjdk.java.net/jdk/pull/7378/files/121811e9..91a2f54a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7378&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7378&range=00-01

  Stats: 8504 lines in 147 files changed: 6385 ins; 1092 del; 1027 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