partition_snapshot_reader: Do not fill buffer in constructor
partition_snapshot_reader: Do not fill buffer in constructor

The reader fills up the buffer upon construction, which is not what other
readers do, and is considered to be waste of cycles, as the reader can be
dropped early.

Refs #1671
test: unit(dev)

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>
Message-Id: <20200910171134.11287-2-c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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