[querier] introduce RepeatList class, used to optimize size of data returned for result set description When rql query has no ambiguity, we used to return the same description * N where N is the size of the result set. Returning RepeatList class avoid that multiplication. According to quick benchmark this improve performance for result set whose size is ~ > 50, has very small penalty for rset < 50, and in any case improve the size of data to be transfered over the network through pyro connection.