bug #50837: add zero-window probe timeout

This commit adds a timeout to the zero-window probing (persist timer)
mechanism. LwIP has not historically had a timeout for the persist
timer, leading to unbounded blocking if connection drops during the
zero-window condition

This commit also adds two units test, one to check the RTO timeout
and a second to check the zero-window probe timeout

src/include/lwip/tcp.h

@@ -310,6 +310,8 @@ struct tcp_pcb {
   u8_t persist_cnt;
   /* Persist timer back-off */
   u8_t persist_backoff;
+  /* Number of persist probes */
+  u8_t persist_probe;
 
   /* KEEPALIVE counter */
   u8_t keep_cnt_sent;