I had a half-built mechanism to track, during the voting process,
whether the Ed25519 value (or lack thereof) reflected a true
consensus among the authorities.  But we never actually inserted this
field in the consensus.

The key idea here is that we first attempt to match up votes by pairs
of <Ed,RSA>, where <Ed> can be NULL if we're told that there is no
Ed key.  If this succeeds, then we can treat all those votes as 'a
consensus for Ed'.  And we can include all other votes with a
matching RSA key and no statement about Ed keys as being "also about
the same relay."

After that, we look for RSA keys we haven't actually found an entry
for yet, and see if there are enough votes for them, NOT considering
Ed keys.  If there are, we match them as before, but we treat them
as "not a consensus about ed".

When we include an entry in a consensus, if it does not reflect a
consensus about ed keys, then we include a new NoEdConsensus flag on
it.

This is all only for consensus method 22 or later.

Also see corresponding dir-spec patch.