This makes the base vk::CommandBuffer class as close to the Vulkan
API as possible. It moves the extra functionality and state tracking
to a CommandBufferAndState helper class.

Also no longer stores a reference to a CommandPool in a CommandBuffer.
Eventually we won't need to free CommandBuffers explicitly, since they
can be freed en-masse by deallocating the CommandPool on a flush.

Bug: angleproject:2200
Change-Id: I5095fe89e8da935ff8273b1a402ccfd6a3ffe0df
Reviewed-on: https://chromium-review.googlesource.com/778184Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>