Message bundling

[fjarri]

We may want to send several messages from one node at the same time:

• To accommodate for one node owning several key shares (see Support one party owning several key shares #31);
• To merge broadcast and direct messages (since technically we don't have a broadcasting facility anyway; although some users may have it, so how do we support both ways?); (this was done in Merge direct and broadcast messages #112)
• To piggyback broadcast consensus messages on the messages from the next round (see Implement piggybacking of echo-broadcasts #114).

A separate question: do we sign the whole bundle (faster), or each message separately (easier to attach only the needed ones to the proofs)?