Gate all access to the queue and caches with mutexes.  Does not handle sharing
of resources in share groups across threads yet.

BUG=angleproject:2464

Change-Id: I297f8f1a535b99efca663cf72bac3d90df8b5d97
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1592253
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>