sstables: Make sstable object not movable
sstables: Make sstable object not movable

Will be easier to add non-movable fields.

We don't really need it to be movable, all instances should be managed
by a shared pointer.

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