Server groups

[HimbeerserverDE]

Allow groups of multiple servers to be defined in the config, likely as a struct field of the Server type as this approach provides the highest level of safety when it comes to dynamic servers. This would make a simple builtin load balancer possible, though #129 should be used instead if there are special requirements.

The following features should take server groups into account:

• Fallback
• Default server
• Hopping (non-raw only)

ClientConn should not get a new exported accessor for the current server group. It will be accessible by using the server name as a key to the server map of the config. The DefaultServer* functions in config.go should be refactored accordingly.

There should probably be chat commands to switch servers by group or send other players to a new server group.

[HimbeerserverDE]

Fallback will be excluded since chaining is possible and more predictable. As for hopping, there's going to be a new HopGroup method. The default Hop method is not going to process groups to keep this feature optional and avoid user confusion.

Please open a new issue if any of the above turn out to be useful.