Combine cache key change and dropping of query strings

https://github.com/Comcast/traffic_control/issues/511

mtorluemke commented on Sep 11, 2015
Not sure exactly how best to do this. Perhaps:

Get rid of the drop_qstring column, and beef up the 'recipes' for the cacheurl field.
Add 'cache_key_string' column, and add smarts to combine the two when both fields are set.