Allow Logging to stdout with separate Error Log Channel (#718)

* Add dedicated error logging writer

* Document new errors to stdout flag

* Update changelog

* Thread-safe the log buffer

* Address feedback

* Remove duplication by adding log level

* Clean up error formatting

* Apply suggestions from code review

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

pkg/apis/options/legacy_options.go

@@ -91,7 +91,7 @@ func (l *LegacyUpstreams) convert() (Upstreams, error) {
 		case "static":
 			responseCode, err := strconv.Atoi(u.Host)
 			if err != nil {
-				logger.Printf("unable to convert %q to int, use default \"200\"", u.Host)
+				logger.Errorf("unable to convert %q to int, use default \"200\"", u.Host)
 				responseCode = 200
 			}
 			upstream.Static = true