postgres / db-datanommer: use lock-wrapper for backups
    
    It seems sometimes now db-datanommer is taking more than 24hours to
    backup, and currently that means it starts another one while the
    previous one is running. Thats no good for anyone, so lets put in a lock
    wrapper to avoid that.
    
    Signed-off-by: Kevin Fenzi <kevin@scrye.com>