QuorumSize is not correctly implemented for 3f+3 case.

[hanish520]

Currently, QuorumSize is implemented as

func NumFaulty(n int) int {
	return (n - 1) / 3
}

func QuorumSize(n int) int {
	return n - NumFaulty(n)
}

QuorumSize from this implementation may return a number higher than the required.