Adx user properties fix

I suggest the following two patches to the adx exchange connector:

1. Store the `cookie_age_seconds` value in the `user.ext` field which has become available in OpenRTB 2.1 and which has been merged to master recently.
2. Store the hex-representation of `hosted_match_data` as `user.buyeruid`, since the canonical use case for `hosted_match_data` is cookie-matching where on other exchanges one usually just drops the own user id in the exchange's match tables.