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.

httpd/httpd.h

@@ -12,6 +12,9 @@
    for each TCP connection. */
 typedef struct httpd_state {
    uint8_t tstate;
+   uint8_t outbuf[2048];
+   uint16_t slen;
+   uint16_t o_idx;
 } uip_tcp_appstate_t;
 
 /* Finally we define the application function to be called by uIP. */