[Reviewer: Mike] Sprout changes for P-Associated-URI and P-Called-Party-ID

Mike, please can you review my sprout changes for P-Associated-URI and P-Called-Party-ID?

The changes are

* removing authentication options - it's now always turned on (and always uses IMS-style) on sprout, and disabled on bono
* querying homestead on registration to find the "primary" associated URI
* storing bindings on this URI in memcached
* sending back all the URIs in the P-Associated-URI header
* querying homestead on INVITEs and looking this URI up in memcached
* when rewriting the request URI, storing the old request URI to P-Called-Party-ID if the target was from memcached.

I've added UT for all this, run the automated live tests and tested live.

There is one bug I'm aware of: we should be wrapping the P-Associated-URI and P-Called-Party-ID as name-addrs.  I'm hoping to fix this shortly, but it should be pretty self-contained.  i hope that's OK.