Reads ClientStats for a given client and time range.

def ReadClientStats(self, client_id,
                      min_timestamp,
                      max_timestamp
                     ):
    """Reads ClientStats for a given client and time range."""
    results = []
    for timestamp, stats in iteritems(self.client_stats[client_id]):
      if min_timestamp <= timestamp <= max_timestamp:
        results.append(stats)
    return results