sstables: optimise clustering rows filtering
sstables: optimise clustering rows filtering

Clustering rows in the sstables are sorted in the ascending order so we
can use that to minimise number of comparisons when checking if a row is
in the requested range.

Refs #1544.

Signed-off-by: Paweł Dziepak <e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>
Reviewed-by: Piotr Jastrzebski <2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>
Message-Id: <1471608921-30818-1-git-send-email-e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>

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