Default server is incorrect sometimes

[HimbeerserverDE]

This is because looping over a map doesn't guarantee anything about the order. We can either use a config key to set the default server or use reflection to get the first entry if it is possible.

[HimbeerserverDE]

Strategy:

• Add a new mandatory config option for the default server (server name, string)
• Fail if the option is unset or references an inexistent server
• If last server is unavailable: inform client: fallback rather than using the default
• If default is unavailable: fallback (should be working already, needs testing)

See #129 and #142 for details on load balancing and related features.