lib/connections, lib/model: Track last connection duration (ref #7223) (#7242)

This adds a statistic to track the last connection duration per device.
It isn't used for much in this PR, but it's available for #7223 to use
in deciding how to order device connection attempts (deprioritizing
devices that just dropped our connection the last time).

lib/connections/relay_dial.go

@@ -63,7 +63,7 @@ func (d *relayDialer) Dial(ctx context.Context, id protocol.DeviceID, uri *url.U
 		return internalConn{}, err
 	}
 
-	return internalConn{tc, connTypeRelayClient, relayPriority}, nil
+	return newInternalConn(tc, connTypeRelayClient, relayPriority), nil
 }
 
 type relayDialerFactory struct{}