whisper: interface changed to simplify the transition to v5

* whisper: mailserver test introduced, refactoring

* whisper: validation test updated

* whisper: max number of peers fixed

* whisper: verification bug fixed

* whisper: esthetic fix

* whisper: interface changed to simplify the transition to v5

* whisper: preparation for version switch

cmd/wnode/main.go

@@ -198,10 +198,11 @@ func initialize() {
 				utils.Fatalf("Failed to read Mail Server password: %s", err)
 			}
 		}
-		shh = whisper.NewWhisper(&mailServer)
+		shh = whisper.New()
+		shh.RegisterServer(&mailServer)
 		mailServer.Init(shh, *argDBPath, msPassword, *argServerPoW)
 	} else {
-		shh = whisper.NewWhisper(nil)
+		shh = whisper.New()
 	}
 
 	asymKey = shh.NewIdentity()