allow per-partition LIMIT clause in cql

somewhat related to static columns (#6561) and slicing (#4851), it is desirable to apply a LIMIT on a per-partition rather than per-query basis, such as to retrieve the top (most recent, etc) N clustered values for each partition key, e.g.

– for each league, keep a ranked list of users
create table scores (league text, score int, player text, primary key(league, score, player) );

– get the top 3 teams in each league:
select * from scores staticlimit 3;

this currently requires issuing one query per partition key, which is tedious if all the key partition key values are known and impossible if they aren't.