frontend: log pending build records only when debugging
    
    Today we almost ran out of disk space on copr-frontend because we are
    mass-rebuilding rubygems, and periodically logging all pending builds
    resulted in 20G `copr-frontend.log`.
    
    I think we can simply log the number of pending builds instead of
    their whole records. That might be useful for debugging though, so I
    am keeping the original command but switching it from `log.info` to
    `log.debug`.