frame/authority-discovery: Have authorities() return both current and next

Authority address lookups on the DHT happen periodically (every 10
mintues) and are rather slow (~10 seconds).

In order to smooth the transition period between two sessions, have the
runtime module return both the current as well as the next authority
set. Thereby the client authority module will:

1. Publish its addresses one session in advance.

2. Prefetch the addresses of authorities of the next session in advance.