- ``get_verstring``, ``get_caps`` are now coroutines.
- ``assign_verstring``, ``cache_caps`` now return a Future.

side-effect: fix supports() and has_identity() broken since forever

Fixes #3455

This reuses the 'to' attribute of the received presence as a 'from' for
the Disco IQ. I don't know if this is the best way to do it, but in
testing with a simple component it fixed the error.

Also I'm not sure whether pres['to'] would also be valid when
is_component is False.

form['fields'] is an ordered list of fields while most plugins expect a
dict there. Fixes, among other things, a caps bug.

(and fix caps in the process)

This is a big-and-dirty commit with a bunch of cleanup, maybe breaking a few
things, and not fixing all iq.send() calls yet.

This makes updating the config after plugin initialization much easier.

To ensure that disco info, or any settings which depend on the bound
JID, are correct, only set such information on or after the
session_bound event has fired.

The API registry generalizes the node handler system from the xep_0030
plugin so that other plugins can use it.

As part of adding this feature:

    - fixed bug in update_caps() not assigning verstrings
    - fixed xep_0004 typo
    - can now use None as a roster key which will map to boundjid.bare
    - fixed using JID objects in disco node handlers
    - fixed failing test related to get_roster

Several of these bugs I've fixed before, so I either didn't push them
earlier, or I clobbered something when merging. *shrug*

Protip: Don't test using a custom disco handler that always returns the
same feature set :p

Finally