RFR: Add generations to freeset [v7]

[Kelvin Nilsen]

> ShenandoahFreeSet has not yet been modified to deal efficiently with the combination of old-gen and young-gen collection set reserves.  This PR makes changes so that we can distinguish between collector_is_free, old_collector_is_free, and mutator_is_free.  Further, it endeavors to keep each set of free regions tightly packed, so the range of regions representing each set is small.
> 
> In its current form, this no longer fails existing regression tests (except for known problems that are being addressed independently)

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Respond to reviewer feedback
  
  Various improvements suggested by reviewers.  Mostly improved comments
  and some minor refactoring.

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/250/files
  - new: https://git.openjdk.org/shenandoah/pull/250/files/8cfbb7fe..b0449e89

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=250&range=06
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=250&range=05-06

  Stats: 319 lines in 6 files changed: 130 ins; 44 del; 145 mod
  Patch: https://git.openjdk.org/shenandoah/pull/250.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/250/head:pull/250

PR: https://git.openjdk.org/shenandoah/pull/250