We do need to hit the registration store every time, unfortunately, because we need to set the registration state as a parameter on the P-Served-User header we issue. Furthermore, I think most iFCs will specify whether they're originating or terminating - but the only way to do that is to say something like ORIGINATING_REGISTERED || ORIGINATING_UNREGISTERED. Avoiding the registration lookup in this case gets trickier.

However, given that the caller needs to know the reg state to build the message it sends, I will lift the is_registered logic out of IfcHandler into stateful_proxy.