Add interface type-checking for timeSource config in RequestDelay.

Use MustBe.instanceOf() to validate that non-null timeSource values
implement IntfTimeSource, matching the pattern used in TokenBucket.

Author: Jeremy Bornstein

src/webapp-builtins/export/RequestDelay.js

@@ -101,12 +101,9 @@ export class RequestDelay extends BaseApplication {
        * @returns {IntfTimeSource} Accepted configuration value.
        */
       _config_timeSource(value = null) {
-        // TODO: Check that a non-`null` `value` actually implements
-        // `IntfTimeSource`.
-
         return (value === null)
           ? StdTimeSource.INSTANCE
-          : MustBe.object(value);
+          : MustBe.instanceOf(value, IntfTimeSource);
       }
 
       /** @override */