Create a locked version of the load_buffer* load_image* family of functions

[brandonpollack23]

It sucks that every buffer/image loaded (such as uniform buffers which are loaded one after another for each target). have to relock every time.

There should be a version of these functions that acts on an already locked buffer_image_store.

OR

Also consider start_load_batch and return proxy object that is an Arc to the unlocked store write handle and do writing to that object and then close it. This could also be a public api, and could be used to solve the performance issue of waiting for idle for every load. The completion can release the lock and wait for idle.