Fix for catchup-only.
Previously when the get_db call succeeded the result of this was not
copied to head.db. This results in incomplete data on this specific
node. Now the get_db call will download it to head.db.clone, and when
that is finished do an inplace inject-as-head with this file. After that
the main database is removed after which the head is copied in to place,
followed by the catchup (as before).