memory: make the allocator refcounted

memory: make the allocator refcounted

Add refcounting to the GstAllocator object.
Remove const from functions because the allocator is refcounted now.
Rename the vmethods for consistency
Expose the constructor for GstAllocator and add a destroy notify for the
user_data. This should make it possible to create allocators that are not
registered and shared globally along with the possibility to destroy them
properly.
Update defs with new symbols.