Reduce large retransmission timeout

The default settings in lwIP TCP implementation result in a very long retransmission timeout of about 20 minutes. The current setting is 12 retransmissions while the TCP backoff mechanism (required by TCP specification) doubles the delay between two subsequent retransmissions  up to factor 512 the round trip time. Retransmissions 6 to 12 are thus delayed by about 100 seconds each on a unidirectional connections (unidirectional connections have round trip of about 200ms due to delayd ack mechanism)