mod_{compression/disco/register/roster/saslauth/tls}: attempt implementing XEP-170,
The XEP specifies guidelines about the order of stream features and is regarded
as security critical possibly so it's best to have it.
Basically in Metronome we just adjust event callback priority to issue the correct
order. (Fixes #42)
mod_{disco/roster}: roster ver and caps should always be at the bottom.
mod_{compression/dialback/saslauth}: adjust order a bit more.