partition_snapshot_reader: Use on-demand comparators
partition_snapshot_reader: Use on-demand comparators

There are already two raii-sh comparators on reader, next patch will
need to add the third. This just bloats the reader, the comparators
in question are state-less and can be created on demand for free.

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

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