Implement piggybacking of echo-broadcasts

[fjarri]

Currently echo-broadcasting takes a separate round. As suggested in the CGGMP paper, these messages can theoretically be attached to the messages from the next round.

I see two problems that may arise:

1. This would work for CGGMP protocols, but not for WWW02 where there is echo-broadcasting in the last round.
   Possible solutions:
   • add a "dummy" round to WWW02;
   • make the echo-broadcasting system flexible enough to be able to run in a separate round when necessary.
2. With the current framework assumptions, the message destinations in the next round may be different from the message destinations in the round that requires echo-broadcasting. Possible solutions:
   • support merging the destinations from two rounds and create messages accordingly (some will have only round messages, some will have echo messages, some will have both). That's a lot of logic though.

Note that all the problems are created by WWW02 (key resharing) since it's the only protocol where not all nodes send messages, and not all nodes receive them. Can we somehow isolate it? Or implement echo for it manually?

[fjarri]

Discussion moved to entropyxyz/manul#8