CommandPool reset and trim

CommandPool reset and trim

Implemented vkResetCommandPool and vkTrimCommandPool.
The reset command out all command buffers in the command
pool back to the initial state. It also frees up the entire pool.

The trim command is a potential future memory usage optimization.
It is currently a noop.

Passes a few tests in dEQP-VK.api.command_buffers.

Bug b/119827933

Change-Id: Ic974f63a8dfca014462ac904218b9dcc3035fc8a
Reviewed-on: https://swiftshader-review.googlesource.com/c/24370
Tested-by: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>