This reduces contention on the mutex protecting the {CompilationState}
by splitting the compilation unit queues into several queues (one per
background task). Each task executes its own queue first, and steals
from other queues once it runs out of work.
The implementation of the set of work-stealing queues is encapsulated
in the new {CompilationUnitQueues} class in module-compiler.cc.

R=titzer@chromium.org

Bug: v8:8916
Change-Id: I5a40314917e7d4a35d7ff9e8ec124ec212beacab
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1543350
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60572}