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

[William Kemper]

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

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Commit messages:
 - 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

Changes: https://git.openjdk.org/shenandoah/pull/398/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=398&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325808
  Stats: 1169 lines in 12 files changed: 634 ins; 495 del; 40 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