multiport: carry a lost install race's counter to the surviving lane window

Two routines can race on a lane's first packet and derive a session each. Only
one gets installed, and the loser was dropped along with the replay-window entry
for the packet it had just accepted, leaving that one counter replayable.

The two sessions hold identical keys, so the winner's window is the same window
in every respect that matters: mark the counter seen on it instead. Closes the
gap without putting a lock on the decrypt path.

outside.go

@@ -177,7 +177,7 @@ func (f *Interface) readOutsidePackets(via ViaSender, packet []byte, rxc *rxCont
 	if !laneCached {
 		// The packet decrypted, so the peer really is using this lane and the
 		// session we derived for it is worth keeping.
-		hostinfo.lanes.installSession(lane, ci)
+		hostinfo.lanes.installSession(f.l, lane, ci, h.MessageCounter)
 	}
 
 	// Roam before we respond, but only on the base tunnel: a lane's source