Code LGTM. Left a few comments for improvements.

Everything seems to work fine, except for leaving channels:

  sockethub:middleware:message Error: platform type leave not supported by xmpp platform. (types: credentials, connect, update, send, join, query, request-friend, remove-fri
end, make-friend)                                                                                                                                                            
  sockethub:middleware:message     at Array.<anonymous> (/home/basti/src/sockethub/sockethub/packages/server/dist/middleware/validate.js:46:33)  

That's actually really cool that it fails "cleanly" with leave because the platform schema doesn't define it in it's schema. Because it shows that the work in tightening up the schema validation works (eg. it doesn't fail unexpectedly and it doesn't sloppily succeed when it wasn't defined in the schema).

@raucao all your points have been addressed, ready for review again.