consensus: Update Certificate and Committee-related terminology

[goshawk-3]

Summary

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

Certificates:

• Attestation: is a collection of votes (both Validation and Ratification votes)
  • A Valid Attestation is an Attestation containing a quorum of Valid votes
  • A Failed Attestation is an Attestation containing a quorum of non-Valid votes
• Certificate: it includes two Valid Attestations (the Validation one and the Ratification one)
  • It is produced by the block generator to certify the previous block

Voting Committees:

• Each committee member is assigned one or more credits (votes that can be cast)
• Committee Size (the number of votes in the committee) is replaced by Committee Credits

MRB:

• Rename mrb (most recent block) to tip to be consistent with the rest of the blockchain space.

Context

This issue is addressing second half of #1178

[fed-franz]

I've updated the description to be in line with recent changes