Create the CommandBase class from Command

The Handle() method is not part of CommandBase, making it possible to provide a different Handle() in each class derived from CommandBase
m_spanningtree Move all server-to-server command handlers into handler classes

These commands are not registered in or called by the core. To find the handler for a command the new server-to-server command table is searched first and if there is no handler (meaning the command isn't a special s2s command), then the core is consulted, as before.