layers: Tuning MemTracker

layers: Tuning MemTracker

Reverse sense of fenceMap using VkFence instead of fenceID as key.
 This eliminates iterating over all of fenceMap.
Stop creating internal fences and let vkQueueSubmit pass NULL fence.
Track lastSubmittedFence and lastSubmittedQueue in command buffer info.
Use unordered_map instead of map for better performance.
Reuse iterator returned by map find whenever available.
 It is faster than repeating a find operation with "[]" notation.
Remove map entries at time of VkDestroy instead of other calls such as Wait.