code: Rework vote extensions

[romac]

The way vote extensions work right now is very cumbersome and to be frank quite confusing.

A vote extension is currently added to a precommit by looking up the corresponding full proposal assembled by the application and copying the extension found there into the precommit. This complicates the logic and mixes up the vote extension process with the building of the proposal. Moreover, and because that process is so twisty, I am not even sure that this is implemented either fully or correctly at the moment, cf. this TODO.

Instead we should add a dedicated effect to the core consensus library which would be relayed to the application, allowing to explicitly extend its vote if it chooses to.

[cason]

In addition to the architectural comments, I would add that send arbitrary-size vote extensions should use a different dissemination network. Vote extensions are payloads, they should be broadcast using communication channels that are more concerned with high throughput and low duplication than strong reliability and low latency.