RFR: 8252888: Simplify G1MMUTracker class hierarchy [v2]

[John Tortugo]

> Relates to: https://bugs.openjdk.java.net/browse/JDK-8252888
> Tested on: x86_64 - Linux - tier1
> Need sponsor: yes
> 
> Merge `G1MMUTracker` and `G1MMUTrackerQueue` into a single class and make method members non virtual.

John Tortugo has updated the pull request incrementally with one additional commit since the last revision:

  Rename MMUTrackerQueue[Element] to MMUTracker

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/214/files
  - new: https://git.openjdk.java.net/jdk/pull/214/files/06acb247..136a1ae7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=214&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=214&range=00-01

  Stats: 534 lines in 7 files changed: 263 ins; 263 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/214.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/214/head:pull/214

PR: https://git.openjdk.java.net/jdk/pull/214