Use multiple concurrent TCP connections

This uses multiple concurrent TCP connections by making the state
of the connection including the output buffer a part of the application
data. This leads to the buffer, to be sent length of data and the already
sent data to be part of that state and must be handed over to functions
that generate data. At this point this leads to an overuse of OSEG and DSEG
memory space so further work needs to be done on tuning the code.
Putting it into a branch for now.

uip/uip-conf.h

@@ -94,14 +94,14 @@ typedef unsigned short uip_stats_t;
  *
  * \hideinitializer
  */
-#define UIP_CONF_MAX_CONNECTIONS 1
+#define UIP_CONF_MAX_CONNECTIONS 3
 
 /**
  * Maximum number of listening TCP ports. TODO: increase this!
  *
  * \hideinitializer
  */
-#define UIP_CONF_MAX_LISTENPORTS 1
+#define UIP_CONF_MAX_LISTENPORTS 3
 
 /**
  * uIP buffer size.