The XDR encoder doesn't understart slices of strings very well. It can
encode and decode them, but there's no way to set limits on the length
of the strings themselves (only on the length of the slice), and the
generated diagrams are incorrect. This trivially works around this,
while also documenting what the string actually is (a URL).
1. Change listen addresses to URIs
2. Break out connectionSvc to support listeners and dialers based on schema
3. Add relay announcement and lookups part of discovery service
