RFR: 8325808: GenShen: Move generational mode code out of shFullGC.cpp [v3]

[William Kemper]

> This change reduces the differences from the upstream branch by moving large chunks of generational mode code into separate files.

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains ten commits:

 - Merge remote-tracking branch 'shenandoah/master' into isolate-full-gc
 - Improve names and comments
 - Fix zero build
 - Initialize member
 - Fix typo, remove TODOs
 - Small cleanup
 - Move some smaller chunks of code out shFullGC
 - Fix warnings
 - Move some big chunks of code out of shFullGC

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Changes: https://git.openjdk.org/shenandoah/pull/398/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=398&range=02
  Stats: 1187 lines in 11 files changed: 650 ins; 496 del; 41 mod
  Patch: https://git.openjdk.org/shenandoah/pull/398.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/398/head:pull/398

PR: https://git.openjdk.org/shenandoah/pull/398