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. When looking for the handler of a command a new command table is searched first which contains all server-to-server commands. If a handler cannot be found in there, the core command table is consulted.
m_spanningtree Minor cleanup of functions