MultiplexingRequestor leaves lingering threads after close() called

The MultiplexingRequestor class does not have a close() method to close the ScheduledThreadPoolExecutor. This leaves non-daemon threads hanging around and thus applications will not quit until System.exit() is called (causing Tomcat applications exporting Lingo services and standard Java apps calling Lingo services to hang forever on shutdown).

Attached is an updated MultiplexingRequestor class that fixes the issue.