vkDestroyCommandPool has worse performance than reset. Instead of
destroying and creating new command pools for every batch of work we
can instead recycle unused command pools. This seems to improve
performance on most systems.

Bug: angleproject:3489
Change-Id: I7f8fe7f9385ca481e04f226a26426c3e92da19ff
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1636410Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>