Prepare partition_snapshot_flat_reader
Prepare partition_snapshot_flat_reader

This commit creates a copy of partition_snapshot_reader
and names it partition_snapshot_flat_reader.
This new class will be turned into a flat_mutation_reader
in the next commit.

The purpose of this commit is to make it easier to review the next
commit.

Signed-off-by: Piotr Jastrzebski <2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>

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