implement a fix for CO15

[themighty1]

The base OT protocol CO15 used in mpz is known to have a broken security proof.
In this discussion emp-toolkit/emp-ot#74 it was suggested to use a fix from https://eprint.iacr.org/2018/499
But that fix (see APPENDIX A) introduces a selective failure attack.

We need to investigate if the selective failure attack is acceptable within the context of the TLSNotary protocol.
If it is acceptable, we need to implement the fix.