Adds an initial implementation of a concurrency support for the string
table, allowing it to be read without holding a lock, and written to
while holding a lock.

This is an initial prototype of _roughly_ how the concurrency would
work; there are still a few holes (e.g. around deserialization). This
is predominantly to assess the main-thread runtime impact of the more
complex string table access.

Bug: v8:10729
Change-Id: I5c6c35e6fca309efd6ee79804c16972aae1ab3ab
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2306804Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68985}