Fix how tailable cursors work. For context: GODRIVER-125.

In particular, Cursor.Next should return false iff

• an error occurred
• the cursorId in the response is 0 and there are no documents left in the current batch of results

Otherwise, it should call the getMore command in a loop until at least one document is returned in the next batch.

Note: this should be done for all cursors, not just tailable. There's nothing in the wire protocol contract that says the server can't return an empty batch with a non-zero cursorId.

Also, consider that the initiating command (e.g. find, aggregate) can return an empty first batch.