Return a method that logs progress/mem usage.

def build_usage_logger(self, log):
        '''Return a method that logs progress/mem usage.'''
        def _log_usage():
            log.debug("{}mb peak memory", peak_memory())
            log.debug("{} pending alignment pairs; {} peak pairs",
                      self.pending_pair_count,
                      self.pending_pair_peak_count)
        return _log_usage