fix repeated initialization of outgoingPathManager in Conn.AddPath (#5280)

In case of concurrent calls, this may still be called multiple times.

Co-authored-by: Marco Munizaga <git@marcopolo.io>

path_manager_outgoing.go

@@ -128,6 +128,9 @@ type pathManagerOutgoing struct {
 	pathToSwitchTo *pathOutgoing
 }
 
+// newPathManagerOutgoing creates a new pathManagerOutgoing object. This
+// function must be side-effect free as it may be called multiple times for a
+// single connection.
 func newPathManagerOutgoing(
 	getConnID func(pathID) (_ protocol.ConnectionID, ok bool),
 	retireConnID func(pathID),