[IBFT] Change quorum size calculation

[dbrajovic]

Description

This PR replaces the old calculation of finding the minimum number of validators reaching quorum.

N - number of validators in network
F = floor((N-1) / 3) - number of (allowed) faulty nodes in a given network

Previously :

quorum = 2 * F + 1

Now:

quorum = ceil(2/3 * N)

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

Manual tests

Please complete this section if you ran manual tests for this functionality, otherwise delete it

Additional comments

Fixes EDGE-514

The formula (previously used) for calculating the Quorum size, which is 2F + 1 did not scale well with the number of validators in a given network. Note the following differences:

2F + 1:

• N = 1, F = 0 -> Q = 1
• N = 2, F = 0 -> Q = 1
• N = 3, F = 0 -> Q = 1
• N = 4, F = 1 -> Q = 3
• N = 5, F = 1 -> Q = 3
• N = 6, F = 1 -> Q = 3
• N = 7, F = 2 -> Q = 5
• N = 8, F = 2 -> Q = 5
• N = 9, F = 2 -> Q = 5

ceil(2/3 * N):
from table (for N=3 the formula does not apply, so a special if statement was used)

• N = 1, F = 0 -> Q = 1
• N = 2, F = 0 -> Q = 2
• N = 3, F = 0 -> Q = 3
• N = 4, F = 1 -> Q = 3
• N = 5, F = 1 -> Q = 4
• N = 6, F = 1 -> Q = 4
• N = 7, F = 2 -> Q = 5
• N = 8, F = 2 -> Q = 6
• N = 9, F = 2 -> Q = 6

[Kourin1996]

@dbrajovic Can you write down the reason so that we can look back in the future?

[zivkovicmilos]

Looks good 💯

I've went through the code and the spec, along with Saltini's paper, the logic seems to be sound.
I don't like that the formula differs for N < 4, but 🤷‍♂️ .

[Kourin1996]

LGTM!