Some of the terminology in Yts is pretty confusing:
* YtsClient is not the opposite of a YtsService.
Suggestion: rename to Context, Connection, Session or something.
* YtsCapability seems to mean "thing with one or more capability strings
(FQC IDs)". Suggestion: rename to Capable or Service or something.
* It's not always clear what "service" means. Depending on context, it
can be a service that we advertise to others, or a service that others
advertise to us. Suggestion: rename methods relating to our own services
to things like yts_client_foreach_advertised_service().