Fixes a bunch of memleaks related to the new Machine and VFS
architecture.
The Machine and vfs_node objects were created with 'new' but were never
properly destroyed, leading to memleaks during CoW operations and on
server shutdown.
- Added a recursive 'delete_vfs_tree' function
- Machine destructor calls this function to lean up its VFS
- CommandProcessor correctly deletes old Machine objects after a CoW
- Player destructor cleans up the player's home machine.
- NetworkManager destructor cleans up all world machines.
Old vfs_node was getting a bit big for its boots, trying to be a
filesystem, network interface and the whole damn computer all at once.
This server-side refactor introduces a 'Machine' class that rightfully
owns the VFS, network services and other machine-state sh.t.
