(TK-110) Added idle timeout setting to proxy 

The one other time we added a timeout-related setting to Jetty9 was `shutdown-timeout-seconds`, https://github.com/puppetlabs/trapperkeeper-webserver-jetty9/blob/master/doc/jetty-config.md#shutdown-timeout-seconds.  Even though that timeout was possible at the Java layer to specify in milliseconds, @cprice404 argued that it would be better to not have this be any more precise than seconds.  We may want to do the same here, calling this `idle-timeout-seconds` and multiplying whatever was set in the options by 1000.