We should cache OCSP responses so we don't have to send HTTP requests for every connection that does not have a stapled response.

See https://github.com/mongodb/specifications/commit/895fed92d3aa4daa78dc2f941851a1bc1b72392c for details about the cache key and maintenance policy.

EDIT: The spec has also been updated to add a URI option to opt out of reaching out to OCSP responders. I've updated the title of this ticket to reflect that it includes the URI option changes as well.