If we add `CandidateDestination.candidates` to the `set` first, followed by verified candidates until we have either `node_count` or `node_count + len(CandidateDestination.candidates)` candidates to send the message to (or we run out of verified candidates).

This would result in more consistent behaviour because the random choices made in `dispersy_yield_verified_candidates` is not a factor any more.  This will also make it possible to make the unit tests test more deterministic, since we know exactly how many candidates should receive a message.