This will prevent indefinitely-hanging requests in case some service or middle box is malfunctioning.
Fix vet errors and lint warnings
Add vet to CI check
Only get issuer certificate if it would be used
No need to make a GET request if the OCSP server is not specified in leaf certificate
Fix CI tests
Make tests verbose
* new ChallengeProvider with Present and CleanUp methods
* new Challenge type describing `http-01`, `tls-sni-01`, `dns-01`
* new client.SetChallengeProvider to support custom implementations
