APIs of `QueryHandle` don't always make stateful behavior clear, yielding sometimes surprising results

[jleibs]

The QueryHandle represents a stateful stream of results including operations like:

• next_row
• seek_to_index_value

However APIs like batch_iter(), built on top of these APIs don't make this stateful behavior clear.

This creates situations where a user can create two batch-iters, which intuitively should each operate independently, but actually step on each other.

[jleibs]

Can we drop batch_iter() all together and only expose into_batch_iter()?