s3: Enforce a lock order in dbwrap

s3: Enforce a lock order in dbwrap

This makes sure we do not deadlock from doing two dbwrap_fetch_locked in two
processes in different orders. At open time, we assign a strict order to all
databases. lock_order 1 will be locked first, lock_order 2 second. No two
records of the same lock order may be locked at the same time.