RFR: Enforce maximum capacity of young generation when allocating

[William Kemper]

Also log young generation maximum capacity during initialization.

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Commit messages:
 - Enforce maximum capacity of young generation when allocating

Changes: https://git.openjdk.java.net/shenandoah/pull/93/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=93&range=00
  Stats: 16 lines in 2 files changed: 14 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/93.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/93/head:pull/93

PR: https://git.openjdk.java.net/shenandoah/pull/93