Trying to use the newer ->sqlite_backup_to_dbh method doesn't
seem worth it, as we'll have to support DBD::SQLite <= 1.60
another decade or more.
Dumping 'msgmap-XXXXXXX' into $INBOX_DIR can appear a bit
confusing to users, so give it a "mm_tmp-$PID-XXXXXXXX" name
to emphasize it's a temporary file tied to a given PID.
We also don't want to penalize read-only daemons with
loading File::Temp, so do it lazily.