stables: fix skipping partitions with no rows
stables: fix skipping partitions with no rows

If partition contains no static and clustering rows or range tombstones
mp_row_consumer will return disengaged mutation_fragment_opt with
is_mutation_end flag set to mark end of this partition.

Current, mutation_reader::impl code incorrectly recognized disengaged
mutation fragment as end of the stream of all mutations. This patch
fixes that by using is_mutation_end flag to determine whether end of
partition or end of stream was reached.

Fixes #1503.

Signed-off-by: Paweł Dziepak <e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>
Message-Id: <1469525449-15525-1-git-send-email-e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>

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