RFE: DNS-based discovery via TXT records

[glpatcern]

Following the discussion in #134, there is some consensus that a DNS-based discovery via TXT records is a useful add-on to the protocol and can provide an alternative to the Invitation flow.

However the idea is to keep it for a future 2.0 version of the protocol, as to make it useful it would need to be enforced, which is a breaking change with respect to the current 1.x implementations.

This issue is then to keep track of that decision.