Eliminate use of sun.nio.ch.DirectBuffer in javafx.media

The javafx.media module has a dependency on sun.nio.ch.DirectBuffer and jdk.internal.ref.Cleaner, both of which are internal classes. The following code is to blame:

        public void closeConnection() {
            ...
            if (buffer instanceof DirectBuffer) {
                ((DirectBuffer) buffer).cleaner().clean();
            }
        }

In an effort to eliminate the use of internal packages from core module by javafx.* modules we need an alternative solution.