fn: introducing 503 responses for out of capacity case (#518)

* fn: introducing 503 responses for out of capacity case

*) Adding 503 with Retry-After header case if request failed
during waiting for slots.
*) TODO: return 503 without Retry-After if the request can
never be met by this fn server.
*) fn: runner test docker pull fixup
*) fn: MaxMemory for routes is now a variable to allow
testing and adjusting it according to fleet memory sizes.

api/server/error_response.go

@@ -32,6 +32,11 @@ func HandleErrorResponse(ctx context.Context, w http.ResponseWriter, err error)
 		if e.Code() >= 500 {
 			log.WithFields(logrus.Fields{"code": e.Code()}).WithError(e).Error("api error")
 		}
+		if err == models.ErrCallTimeoutServerBusy {
+			// TODO: Determine a better delay value here (perhaps ask Agent). For now 15 secs with
+			// the hopes that fnlb will land this on a better server immediately.
+			w.Header().Set("Retry-After", "15")
+		}
 		statuscode = e.Code()
 	} else {
 		log.WithError(err).WithFields(logrus.Fields{"stack": string(debug.Stack())}).Error("internal server error")