To support multithreading, contexts should manage their own command graphs
and garbage.  This allows safe access to vulkan resources such as command pools
without thread synchronization.

BUG=angleproject:2464

Change-Id: I930149bc9f0793028761ee05ab50b8c0a4dec98a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1516515
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>