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/tcp_dial.go

@@ -57,7 +57,7 @@ func (d *tcpDialer) Dial(ctx context.Context, _ protocol.DeviceID, uri *url.URL)
 		return internalConn{}, err
 	}
 
-	return internalConn{tc, connTypeTCPClient, tcpPriority}, nil
+	return newInternalConn(tc, connTypeTCPClient, tcpPriority), nil
 }
 
 type tcpDialerFactory struct{}