Changed do_choose_write_nodes to choose the most under-utilized nodes.
Added logic to check_is_orphan to handle calls from the most over-utilized node.
Most over-utilized node calls check_is_orphan for all blobs and tags.
Locations to be removed from tags due to deletion of blobs are stored in gc_blobs.
Addresses to be removed are sent from update_tag_body to tag and deleted.
Added list of urls to remove to the blob_update type.