ovbackup should allow doing a -fast integ

Currently, ovbackup does a full integrity check of the backup database file before it compresses the backup. At some sites with large databases, this may be prohibitively slow. ovbackup should allow doing a -fast integ, which just checks the index/structure of the database without checking the actual data blocks. This behavior should be configurable in backups.conf so that sites can customize when to do a full integ and when to do a -fast integ. (backups.conf is a bash script, so sites could include a snippet to only do full integs once a week, for example.)