layers: Set cmd buffer invalid on event destroy
layers: Set cmd buffer invalid on event destroy

When an event is destroyed, set any cmd buffers that it was bound
to to the CB_INVALID state.

Also added getEventNode() helper and made use of it where appropriate.

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