Move the calls for metadata decoding from the cluster
membership management loop to the main loop. This task
was not directly related to the cluster, and was adding
complexity, including the need for multierr structures.
The wireguard is mostly used to compute metadata.
Metadata is now computed by main.go and encoded in
node.go, the cluster only receives a function generating
the binary metadata.
The cornerstone for exchanging data is the node
structure and associated metadata. Moving it to a
separate file (maybe later a separate module) will help
decoupling.
kaiyou