Remove explicit set of memberlist protocol

Memberlist does a full validation of the protocol version (min, current, max)
amoung all the ndoes of the cluster.
The previous code was setting the protocol version to max version.
That made the upgrade incompatible.

Signed-off-by: Flavio Crisciani <flavio.crisciani@docker.com>

libnetwork/networkdb/cluster.go

@@ -107,7 +107,7 @@ func (nDB *NetworkDB) clusterInit() error {
 		config.BindPort = nDB.config.BindPort
 	}
 
-	config.ProtocolVersion = memberlist.ProtocolVersionMax
+	config.ProtocolVersion = memberlist.ProtocolVersion2Compatible
 	config.Delegate = &delegate{nDB: nDB}
 	config.Events = &eventDelegate{nDB: nDB}
 	// custom logger that does not add time or date, so they are not