Improved network messages

The current network messages are simple streams with inefficient storage (a bool takes full two bytes); this causes a lot of compatibility problems everywhere. For example, it is not (easily) possible to extend the WriteCreate() part of a sync message, and it is not possible to change the way data types are transmitted. I propose switching to a (compressed) xml-like format where messages are essentially typesafe dictionaries, mapping ID strings to values. This decouples encoding and message processing and allows arbitrary extensions of messages and efficient syncing where unchanged values are left out of the message.