vinyl: do not use restore callback for starting read iterator source

Apart from restoring the iterator position in case the source changed,
the vy_stmt_iterator_iface->restore callback is also used for starting
iteration in vy_merge_iterator_next_key() even though next_key() can be
used instead. Let's rewrite the function so that it uses next_key()
instead of restore() where appropriate. This will allow us to simplify
restore() by making it handle nothing but iterator restoration.