Fix --fetch protobuf

Closes #717

Review 519 by @mlucy
Merge branch 'next' into sam_etc_batched_writes
Made next_batch use a sanity check that max_size > 0, not a guarantee.
Made arr->get call in next_batch_impl not have a NOTHROW.
Made RECOMMENDED_MAX_SIZE have the value 3 in debug mode.
Made sort_datum_stream_t update data_index properly.