partition_snapshot_reader: Rename cmp to less for explicity
partition_snapshot_reader: Rename cmp to less for explicity

This is less comparator, cmp is used as a sign of tri-compare in this set.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla