WAL-G optimization backup storage by remote-copy APIs

WAL-G supports two backup types: delta(incremental) and full. Delta backup has higher performance on pushing phase, but it is slower on fetch and sometimes user has to do full backup to remove a bunch of old WAL segments accumulated over time.

Nevertheless creation of full backup could take hours on large (several terabytes) databases. To deal with it file uploading time could be reduced by creation of remote copies of unchanged archives from last full backup in cloud storage instead of direct uploading.