Reduce state kept in DocumentServer

[justinfagnani]

Data flow between the editor and document would be simpler if the editor always sent the information needed to fully execute a command or query document state, without any state being kept in the DocumentServer (except for node ID mappings).

This would be a fairly large refactor because of the way CursorManager and PositionWalker work. Currently they require direct node references. We would need a way to track cursor position and request ranges based on a sourceId + offset.