[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.