This gives a stronger ordering on serials than if they're acquired by
the ContextVk. Part of the steps of implementing multithreaded GL on
Vulkan.

Implements a "globalFinish" method in RendererVk that is triggered on
ContextVk destruction. This helped fixed some racy object deletion
situations where the ContextVk could have queued work that uses deleted
objects. Flush all the Contexts before destruction to avoid these
hanging deleted objects.

Bug: angleproject:2464
Change-Id: I244e9bbf6cd47b272c7cbca45b0fb1eb46d626fc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1791268
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>