storage_proxy: add query result row count to brief format
storage_proxy: add query result row count to brief format

Report number of rows in brief reporting format, but only if
we can count them without linearizing result's buffer.

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