runtime: disable the Nagle algorithm entirely.

See also commit feed91d; that commit fixed the test_rpc_timing test,
but caused frequent hangs elsewhere, which were also caused by buggy
Nagle implementation. Just disable this entirely, as with our
explicit buffering it provides no benefit anyway.

artiq/runtime.rs/liblwip-sys/lib.rs

@@ -149,6 +149,7 @@ extern {
     // nonstandard
     pub fn tcp_sndbuf_(pcb: *mut tcp_pcb) -> u16;
     pub fn tcp_so_options_(pcb: *mut tcp_pcb) -> *mut u8;
+    pub fn tcp_nagle_disable_(pcb: *mut tcp_pcb);
 
     pub fn udp_new() -> *mut udp_pcb;
     pub fn udp_new_ip_type(type_: ip_addr_type) -> *mut udp_pcb;

artiq/runtime.rs/liblwip/lib.rs

@@ -549,6 +549,7 @@ impl TcpStream {
             lwip_sys::tcp_recv(raw, Some(recv));
             lwip_sys::tcp_sent(raw, Some(sent));
             lwip_sys::tcp_err(raw, Some(err));
+            lwip_sys::tcp_nagle_disable_(raw);
             TcpStream { raw: raw, state: state }
         }
     }

artiq/runtime/main.c

@@ -177,6 +177,10 @@ u8_t* tcp_so_options_(struct tcp_pcb *pcb) {
     return &pcb->so_options;
 }
 
+void tcp_nagle_disable_(struct tcp_pcb *pcb) {
+    tcp_nagle_disable(pcb);
+}
+
 int main(void)
 {
     irq_setmask(0);