Reduce the amount of redundant COUNT queries

[aliaksandrb]

Hi!

Small proposal for reducing the amount of COUNT queries when the result set is full already. Kinda hits performance.

Currently, the paginator makes COUNT query for each request (unless the :count option provided by user). On some big tables (millions records) with many filters it adds a lot to response time, as while the main result is limited by user defined limit params, like page[:size], the COUNT queries are executed against the whole table.

And it tends to be redundant if the amount of records returned for a query is already less than limit specified (as there won't be any more pages).