Dynamically generate API request stats

Instead of hardcoding the stats argument to lookup, this change
generates it given the list_items that are passed in. By doing this, you
can pass in the list_items in any order, and you can pass extra items
that we don't have stats for, and they'll be reported as missing.

We've also added a small check in process_tickers that confirms that
each list_item is in the row_dict so that it doesn't throw an exception.
(We ran into this while testing that extra unknown list_items could be
passed to lookup safely.)