New message with all known mediators

Mediator can ask relay for all known mediators.

• New data structure for saving own and network mediators and also the corresponding channels
• Exchange of mediator lists and channels in network
• Response for RequestAkkaSystemsStocktakingMessage

This includes implementing a new efficient data structure to store all akka systems and their respective channels so that an interface for the following functionality can be provided:

• requesting the subscribed channels for an akka system
• requesting the participating akka systems for a channel
• requesting the akka systems that are connected to a given peer
• requesting the peer that an akka system is connected to

The data structure should not contain any information about the assignment of Akka system to relay or vice versa, as this would obsule the DHT. Furthermore, this data structure only fulfills the property eventual consistency in order to not interfere with the availability of the relays.