Add ConcurrentAllocator which can be used for concurrent allocation from a background thread in the old space. ConcurrentAllocator doesn't request a GC yet when an allocation fails. This will be implemented in later CLs.

Bug: v8:10315
Change-Id: I81260ebbd8863c143e93aedb93c66d0e7c28bddb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2144066
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67150}