spanner: close row iterators when done

[ecordell]

There were a couple of places where we build an iterator but then don't use the Do consumer.

Any time we don't use the Do consumer, we need to explicitly close the iterator to avoid leaking a session. In all of the places where we weren't already using Do, we were only reading one row anyway, so I just switched to Do for those cases too.

This should fix #1196, but we'll want to some testing to confirm.

From Spanner Docs:

Read returns a RowIterator. You can call the Do method on the iterator and pass a callback:

err := iter.Do(func(row *Row) error {
  // TODO: use row
  return nil
})

RowIterator also follows the standard pattern for the Google Cloud Client Libraries:

defer iter.Stop()
for {
   row, err := iter.Next()
   if err == iterator.Done {
       break
   }
   if err != nil {
       // TODO: Handle error.
   }
   // TODO: use row
}

Always call Stop when you finish using an iterator this way, whether or not you iterate to the end. (Failing to call Stop could lead you to exhaust the database's session quota.)