read_context: add _partition_exists
read_context: add _partition_exists

This new state stores the information whether current partition
represented by _key is present in underlying.

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

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