layers: threading with cmd buffer handle reuse
layers: threading with cmd buffer handle reuse

A driver may reuse a command buffer handle in another thread.
The threading layer needs to remove command buffer information
before calling FreeCommandBuffers.
This addresses part of github issue #719.

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools