Simplifies some of the logic of message queues in d8 and makes sure
to delete any in-flight messages upon worker termination. Drive-by
cleanups of some other small d8 vestiges.

R=clemensh@chromium.org
BUG=v8:9524

Change-Id: I587c0cb3eeed88107e7dba552389057f07c15c43
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1710673
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62873}