Move event types constants into single file

Move all constants for event types to types.go for easier code
readability and maintainance.

Signed-off-by: Zhang Wei <zhangwei555@huawei.com>

supervisor/supervisor.go

@@ -161,7 +161,7 @@ func (s *Supervisor) Events(from time.Time) chan Event {
 		}
 		// Notify the client that from now on it's live events
 		c <- Event{
-			Type:      "live",
+			Type:      StateLive,
 			Timestamp: time.Now(),
 		}
 	}