Keep allocated debug callbacks until destroy instance

Keep allocated debug callbacks until destroy instance

Previously the loader would destroy any debug callbacks created during instance creation
to later create new ones during instance destruction. This required a memory allocation
to occur inside vkDestroyInstance, which can cause leaks if an OOM occurs during
instance destruction.

This commit simplifies the logic by keeping around the allocations made during instance
creation by moving them into their own debug node chain. Then during instance destruction
moves them back.

Also renames several functions to better describe their intended purpose.