Reviewer Ellie: Fixes for when no HSS is configured.

Plumb through hss_configured option to the IMS Subscription handler, so that we don't send SARs when we don't have an HSS.

In other news, we only query the AV cache when we don't have an HSS configured. Previously, we were then trying to send an MAR to the HSS by calling get_av() if the cache didn't return an AV. This obviously wasn't working.