Do not lock mutex for the full duration of the slow path of allocation
on background threads. For example sweeping and allocation of a new page
is now performed outside of the lock.

Bug: v8:10315
Change-Id: Ifee9bc1569d1ec53fdced965bd222dd1eab11b24
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2207131Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67898}