32kB window is actually larger than our typical receive ring size.

Shrink to 4kB for now.  We might want to see if it can be made
genuinely indicative of the RX ring size in the new TCP
implementation.

src/net/uip/uipopt.h

  *
  * \hideinitializer
  */
-#define UIP_RECEIVE_WINDOW   32768
+#define UIP_RECEIVE_WINDOW   4096
 
 /**
  * Determines if support for TCP urgent data notification should be