kick all clients before SocketServer shutdown

Summary: Force a disconnection of all SocketClients before a SocketServer shutdown. The event propagation will allow the SocketServerDelegate to handle anything else it needs to do (in practice the SocketServerDelgate should deal with this before stopping the SocketServer, but there's no assurance that will happen)

Test Plan: shut down server with and without the code. turned off Server disconnect-all code too, made sure everything fired correctly

Differential Revision: https://phabricator.ericstern.com/D12