This is a reland of 6af09b1b

Relanding without changes after fixing the root cause in
https://crrev.com/c/2315987

Original change's description:
> [offthread] Add a write lock to the string table
>
> 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/+/2306804
> Reviewed-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}

Tbr: verwaest@chromium.org,ishell@chromium.org,ulan@chromium.org
Bug: v8:10729
Change-Id: I9ce8882cfbdd40fbe1c7478e171c0785bf2e64d6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2315989
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69025}