Copied pool allocator used by compiler to common and hooking it up as
custom allocator for CommandPools. Modified it to support reallocation.

RendererVk now has a private poolAllocator and VkAllocationCallbacks
struct. The allocation callbacks are initialized to static functions
in RendererVk::initializeDevice() and then passed to CommandPool init()
and destroy() functions.

Using the pool allocator saves Command Pool/Buffer clean-up time which
was showing us as a bottleneck is some cases.

Bug: angleproject:2951
Change-Id: I81aa8a7ec60397676fa722d6435029db27947ef4
Reviewed-on: https://chromium-review.googlesource.com/c/1409867
Commit-Queue: Tobin Ehlis <tobine@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>