RFR: 8252888: Simplify G1MMUTracker class hierarchy

[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.

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Commit messages:
 - Fixing trailing spaces
 - Simplify G1MMUTracker class hierarchy

Changes: https://git.openjdk.java.net/jdk/pull/214/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=214&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252888
  Stats: 264 lines in 5 files changed: 120 ins; 135 del; 9 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