consensus: NilQuorum is inaccurate

[fed-franz]

Summary

Currently, NilQuorum is defined as 1f64 - CONSENSUS_QUORUM_THRESHOLD + 0.01 (ie, as of now, 0.34).
While this works fine with a committee size of 64, it can lead to the NilQuorum to be 1-unit greater than the necessary.

For instance, with a commitee of 120 provisioners, we would have:

• Quorum: 120 * 0.67 = 80.4, which is rounded up to 81
• NilQuorum: 120 * 0.34 = 40.8, which is rounded up to 41

Now, with a Quorum of 81 a NilQuorum should be 40, so we're requiring one vote more than the necessary.

Possible Solution

Set NilQuorum to NilQuorum = COMMITTEE_SIZE - Quorum + 1

[autholykos]

See #1184