Support a way to pass a hint to update command to use a specific index for update query filter. Work done under this ticket will not prevent misuse of hinting, as reported by SERVER-26413.
Change the behavior on the server for update to look for the special fields: $query, $sortby, $limit, $hint. If these are found treat them the same as the Op_Query does.
This will allow sorting (so you can update the right element), $limit (so you can just update some of them), $hint (so you can force the right index) in addition to the normal $query part.
This should be backwards compatible. It will either be a $query + other $ options, or just the query, without options (like it is now).