xmpp: NIH a whole promises library (!), use it for disco#info

- I anticipate we're going to need a promises-like abstraction (like what
  whatsxmpp had), because sending and receiving stanzas is by nature async.
  - So I NIH'd one to avoid having to import blackbird, and we now use it
    for IQ-GET.
- This has the rather nifty effect that we can use the "join" abstraction
  provided to only populate *SERVER-FEATURES* once both IQ-gets (self, server)
  have returned!