rpcserver: Remove unneeded AddedNodeInfo method.

[davecgh]

The RPC server was refactored some time ago to decouple it from the internals of the main server and part of that refactoring modified the RPC server to make use of a local peer interface to obtain all necessary information about peers which is then used to create RPC server results. Due to that, the AddedNodeInfo and PersistentPeers methods now do the exact same thing.

This modifies the rpcserver.ConnManager interface to remove the now unnecessary AddedNodeInfo method in favor of the PersistentPeers method and updates all relevant code accordingly.