Refactor node ID handling, use check digits (fixes #269)

New node ID:s contain four Luhn check digits and are grouped
differently. Code uses NodeID type instead of string, so it's formatted
homogenously everywhere.

protocol/message_types.go

@@ -42,7 +42,7 @@ type Repository struct {
 }
 
 type Node struct {
-	ID         string // max:64
+	ID         []byte // max:32
 	Flags      uint32
 	MaxVersion uint64
 }