RFR: Reserve regions for the collector according to generational reserves

[William Kemper]

When the freeset is rebuilt, it reserves a number of regions for evacuation proportional to the total size of the heap. With default settings, this will reserve more regions than intended for the generational mode and may lead to premature allocation failures for mutators.

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Commit messages:
 - Reserve regions for the collector according to generational reserves

Changes: https://git.openjdk.org/shenandoah/pull/143/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=143&range=00
  Stats: 20 lines in 2 files changed: 16 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/143.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/143/head:pull/143

PR: https://git.openjdk.org/shenandoah/pull/143