Add check on total weight and certificate threshold in verifyValidatorsUpdate

[shuse2]

Expected behavior

Add additional verification logic as defined in the PR

Actual behavior

In LIP0053, in the context of posting a certificate within a CCU, we allow the new certificate threshold to be set to any value. In particular, it can be set to a value less than 1/3 of the total BFT weight or even to 0, thus allowing a following certificate to be signed by a minority of validators not satisfying the BFT requirements.

Steps to reproduce

Which version(s) does this affect? (Environment, OS, etc...)

6.0.0-rc-2