Refactor server-side communications (#11192, #7891)

* Move UIDL writing from AbstractCommunicationManager to UidlWriter
* Move request handling from ACM to RequestHandlers
* Move server RPC message handling to ServerRpcHandler
* Move portlet event notifications to PortletListenerNotifier (a RequestHandler)
* Communication handlers reside in c.v.server.communication

Change-Id: I087e923dbdf88c6b3fcaafbdb7f685d9d3dad0c1