query::result::builder: Add partition count
query::result::builder: Add partition count

This patch adds a partition count to the query::result::builder. It is
intended to be incremented by users, and later used to build a
query::result.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

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