Adding _sessionNotify to server side helloWorld example

The _sessionNotify function has been added to the HelloWorldServer
implemenation. This code shows how to handle events from the stack on the
server implementation side.

Change-Id: Ic3089b2a3b5b9f96f6ef353a4461f04e7c6e844a

git-svn-id: https://svn.apache.org/repos/asf/etch/trunk@1578906 13f79535-47bb-0310-9956-ffa450edef68