Reduce usage of the loader lock in method lookup.

Reduce usage of the loader lock in method lookup.

The function mono_method_inflated_lookup no longer needs the loader lock to be held
as the hashtable is now protected by the imageset lock.

This means that mono_class_inflate_generic_method_full_checked doesn't need to take the
loader lock.

The function mono_method_set_generic_container uses mono_image_property_insert which takes
the image lock and not the loader lock. Adjust the locking comment.