@SuppressWarnings("NullableProblems")
            @Override
            public void put(JRTClientConfigRequest e) {
                // When closed, throw away all responses that callbacks try to put on queue
            }
This method should ideally throw new MissingConfig/Configuration exceptions and let the caller
catch them. However, this would make the code in JRT/File/RawSource uglier.
Alternatively, it could return a SetConfigStatus object with an int and an error message.

@param jrtRequest a config request
@return an instance of a config class (subclass of ConfigInstance)
config/src/main/java/com/yahoo/config/subscription/impl/JRTConfigSubscription.java