Pipelight is leaking a few handles on each plugin execution

Pipelight tries to keep the internal handles of both sides synchronized by taking a look at the commands called and doing some kind of refcounting. Unfortunately it seems like Silverlight and/or the browser does not always release all handles properly. This results in a leak of <= 2 handles on each execution of a Silverlight application. As the browser will shutdown the plugin process after some time anyway there shouldn't be any noticeable problem for users.