- We now compute the number of parallel compaction tasks, depending on the
  evacuation candidate list, the number of cores, and some hard limit.
- Free memory is moved over to compaction tasks (up to some limit)
- Moving over memory is done by dividing the free list of a given space up among
  other free lists. Since this is potentially slow we limit the maximum amount
  of moved memory.

BUG=chromium:524425
LOG=N

Review URL: https://codereview.chromium.org/1354383002

Cr-Commit-Position: refs/heads/master@{#30886}