Instead of dynamically creating semaphore for each page parallel job,
we create one semaphore for MarkCompact and reuse it.

This patch also removes all instrumentation code that was added to
help with investigation.

BUG=chromium:609249
LOG=NO

Review-Url: https://codereview.chromium.org/1998213002
Cr-Commit-Position: refs/heads/master@{#36407}