add hq_user_id to CaseState

http://manage.dimagi.com/default.asp?146765

@czue how embarrassing 

This seemed like the least invasive option. Other options require an extra query to get the user_ids:
* update the `hqcase/by_domain_hq_user_id` so that you can get user_id's from case_id's
* update the SQL case index to include `hq_user_id` column
could you add a comment that this is just for the call center to work and only relevant to user-synced cases? seems like it could throw people off otherwise (the more logical thing this would be is the `user_id` of the case which was what i thought originally)