RFR: Start young collect instead of old if mixed evacuations are pending

[William Kemper]

This change addresses a race condition in which the heuristic could start an old gc while there are pending mixed evacuations. This condition could lead to a crash because the nascent old gen collect will clear the mark bitmap, but the remembered set scan of the bootstrap collection will use the mark bitmap to find objects if it believes mixed evacuations are in progress.

-------------

Commit messages:
 - Start young collect instead of old if mixed evacuations are pending

Changes: https://git.openjdk.java.net/shenandoah/pull/141/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=141&range=00
  Stats: 12 lines in 2 files changed: 11 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/141.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/141/head:pull/141

PR: https://git.openjdk.java.net/shenandoah/pull/141