RFR: Mixed evacuation

[Kelvin Nilsen]

Mixed evacuation

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

Commit messages:
 - Perform mixed collections
 - Perform mixed collections

Changes: https://git.openjdk.java.net/shenandoah/pull/29/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=29&range=00
  Stats: 936 lines in 26 files changed: 811 ins; 43 del; 82 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/29.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/29/head:pull/29

PR: https://git.openjdk.java.net/shenandoah/pull/29