consensus: Update naming convention

[fed-franz]

Summary

With the introduction of #1177, the following naming system will be employed:

• Phase 1 (Selection/Generation) will be called Proposal
  • In the Proposal phase, the Block Generator broadcasts a Candidate message
• Phase 2 (First Reduction) will be called Validation
  • In the the Validation phase, committee members broadcast Validation messages
• Phase 3 (Second Reduction) will be called Ratification
  • In the the Ratification phase, committee members broadcast Ratification messages
  • If a quorum of Ratification votes is reached, a Quorum message is produced

With the introduction of failed-iteration "certificates", the following naming system will be employed:

• Attestation: is a collection of votes (both Validation and Ratification votes)
  • A Quorum Attestation is an Attestation containing a quorum of votes
  • Other Attestations will be used to prove a missed Quorum on a failed iteration
• Certificate: it includes two Quorum Attestations (the Validation one and the Ratification one)
  • It is produced by the block generator to certify the previous block

Following discussions on Voting Committees (see #905), the following naming system will be introduced:

• Each committee member is assigned one or more ballots (or voting credits)
• Committee Size (the number of ballots in the committee) is replaced by Voting Pool (or credits pool)

[fed-franz]

While at this, I'd suggest to rename mrb (most recent block) to tip to be consistent with the rest of the blockchain space.