Updated messageHandler to be a single instance per web server

Updated messageHandler to be a single instance per web server,
Added session manager mock class
Making this a property gives the false feeling that the message handler is exposed to external use. Since ```WebServer``` is set up to respond to client requests, it seems like ```MessageHandler``` will always be required. Why don't we initialize ```this.messageHandler``` in the constructor of ```WebServer``` and use it within the class?