more accurate representation about bft fault number and quorum

[NathanBSC]

Rational:
in BFT consensus, when v = 3f+1, system can tolerate f evil validators, and quorum is 2f+1
but when n%3 != 1，
f should be less than 1/3v, meaning 1/3v is not included even when it's a integer
so 1/3v or more evil validators can destroy the system
q should be more than 2/3v and 2/3v is included when it's a integer,
so we can express q as “2/3v or more” for more accuration

when v = 21, quorum is 2/3v =14, we use it to replace 15