select_statement: Make sure all aggregate queries use paging
select_statement: Make sure all aggregate queries use paging

Mainly to make sure we respect row limits. Since normal result
generation does not for aggregates.

Fixes #752 

Message-Id: <3b2f3e718a9c9e9a75dde6540b8673d52e2d5776@scylladb.com>

kjniemi/scylla,raphaelsc/scylla,avikivity/scylla,kjniemi/scylla,duarten/scylla,scylladb/scylla,kjniemi/scylla,avikivity/scylla,raphaelsc/scylla,scylladb/scylla,raphaelsc/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla