consensus: Distinguish Invalid vote from Timeout

[fed-franz]

Summary

Currently, Provisioners cast the same vote (a signature over a empty hash) for both an invalid block and a timeout event (unknown candidate).

This prevents determining if the Provisioner received an invalid candidate or simply didn't receive any candidate.

Relevant Context

This change is necessary to decide if a block is invalid (a quorum of Invalid votes is needed) and possibly implement Slashing over the misbehaving Generator.

Possible Solution

• Add a valid field to Reduction messages.
• When casting a vote:
  • if the candidate is unknown (Selection timeout-ed) the block_hash field is empty only (valid can be ignored)
  • if the candidate is known, block_hash contains the block hash (valid can be set to true if the block is valid, and false otherwise)

Alternative solutions can be proposed

[herr-seppia]

See #1183 (comment)