With the CT policy changes, we cancel any outstanding requests in a
group once we've gotten a successful response from any log in that
group. That means various function calls will return early with an error
code indicating cancellation. We want to avoid logging such error codes,
because they were not really errors, they were intentional.
This change introduces a small utility package called "canceled", which
checks for both context.Canceled and the gRPC return code indicating
cancelled.
Daniel McCarney