Display the Maia account username in level-3 log output

[rjl]

At $log_level 3, the debugging output in amavisd-maia currently identifies the mail recipient by his maia_users.id value. It would be easier to diagnose recipient mismatches if the maia_users.user_name value were displayed as well.

In current implementations this would require an additional database lookup, but when query caching is eventually introduced it should be straightforward to store this once in a recipient data structure and retrieve it as desired throughout the session.