Record proposals for live rhododendron sessions in the DB

[rphmeier]

To prevent accidental double-propose when going offline for a short period.

DB holds a mapping equivalent to a HashMap<parent_hash, Vec<(round number, proposal)>>.

When proposing at a round k on top of a given parent hash, check if we already proposed at this round and don't create a new one. Otherwise, place the new proposal in the mapping and commit to disk.

When importing a block on top of parent_hash, clear all recorded proposals based on it as they are no longer relevant.

[rphmeier]

Also useful would be to jump-start the consensus process to a round k.

[rphmeier]

Not as relevant anymore, this is for rhododendron.

[gnunicorn]

Closing as RHD isn't actively developed at the moment and would require a major overhall to get started in the first place. This isn't relevant for now.