RFR: 8253343: Extract G1 Young GC algorithm related code from G1CollectedHeap

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that moves the G1 young collection algorithm containing the collection part out of `G1CollectedHeap` into a new `G1YoungCollector` class.

This change is mostly limited to moving the young collection specific code out; data structures are mostly kept in `G1CollectedHeap` for one reason or another, most of the time just to decrease the amount of code changes.

That will be fixed by upcoming (smaller) changes.

Other random comments:
  * I am aware that the class forward definitions in `g1CollectedHeap.hpp` are unsorted, but they were already and I will fix that later.
  * there will be some cleanup of `G1CollecteHeap` interface: at the moment, to reduce changes, I moved only code that was easy to move, and added back-references to `G1CollectedHeap`. There is a large list of getters for those in `G1YoungCollector`. These will be investigated one by one whether they can be removed by moving other code to `G1YoungCollector`.
  * what I would like to move is at least data like the heap region attribute table, evacuation failure temporary information and others
  * some data will likely move from `G1CollectedHeap` to some class that contains long term state specific to the young collection (e.g. evacuation failure injector, evacuation failure final result?, _bytes_used_during_gc, _gc_timer_stw, _gc_tracer_stw, stw reference processor and related closures) 

So lots of changes ahead, but this is a significant first step to avoid me doing tricky merges over and over again.

Testing: tier1-5

Thanks,
  Thomas

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Commit messages:
 - Minor cleanup
 - Merge master

Changes: https://git.openjdk.java.net/jdk/pull/5333/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5333&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253343
  Stats: 2384 lines in 7 files changed: 1323 ins; 1048 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5333.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5333/head:pull/5333

PR: https://git.openjdk.java.net/jdk/pull/5333