memory: cleanups and improve docs

memory: cleanups and improve docs

Make the fallback copy use the same memory allocator as the original object.
Improve some docs.
Require an alloc function when registering an allocator.
Remove gst_memory_allocator_get_default() and merge the feature in
gst_memory_allocator_find()
Fix locks on the hashtable.
Remove defined but not-implemented gst_memory_span() method.