RFR: 8252888: Simplify G1MMUTracker class hierarchy

[John Tortugo]

On Fri, 18 Sep 2020 18:45:16 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> 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.
>
> I think it's better to use `G1MMUTracker` as the type name and mention its queue implementation in the comments,
> instead of including "Queue" in its name.

@albertnetymk - Thanks for reviewing. I just renamed accordingly to your suggestion.

I also renamed MMUTrackerQueueElement to MMUTrackerElement as it made sense to me. Please let me know if you disagree
and I can revert that.

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