Use Bitcoin Core-style ACK / NACK / Nit language in pull request reviews

[cbeams]

From https://bitcoincore.org/en/faq/contributing-code/#peer-review:

Anyone may participate in peer review which is expressed by comments in the pull request. Typically reviewers will review the code for obvious errors, as well as test out the patch set and opine on the technical merits of the patch. Project maintainers take into account the peer review when determining if there is consensus to merge a pull request (remember that discussions may have been spread out over github, mailing list and IRC discussions). The following language is used within pull-request comments:

• ACK means “I have tested the code and I agree it should be merged”;
• NACK means “I disagree this should be merged”, and must be accompanied by sound technical justification. NACKs without accompanying reasoning may be disregarded;
• utACK means “I have not tested the code, but I have reviewed it and it looks OK, I agree it can be merged”;
• Concept ACK means “I agree in the general principle of this pull request”;
• Nit refers to trivial, often non-blocking issues.