mininode: add an optimistic write and disable nagle

Because the poll/select loop may pause for 100msec before actually doing a
send, and we have no way to force the loop awake, try sending from the calling
thread if the queue is empty.

Also, disable nagle as all sends should be either full messages or unfinished
sends.

This shaves an average of ~1 minute or so off of my accumulated runtime, and
10-15 seconds off of actual runtime.

test/functional/test_framework/mininode.py

@@ -1654,6 +1654,7 @@ class NodeConn(asyncore.dispatcher):
         self.dstaddr = dstaddr
         self.dstport = dstport
         self.create_socket(socket.AF_INET, socket.SOCK_STREAM)
+        self.socket.setsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)
         self.sendbuf = b""
         self.recvbuf = b""
         self.ver_send = 209
@@ -1792,7 +1793,14 @@ class NodeConn(asyncore.dispatcher):
             tmsg += h[:4]
         tmsg += data
         with mininode_lock:
-            self.sendbuf += tmsg
+            if (len(self.sendbuf) == 0 and not pushbuf):
+                try:
+                    sent = self.send(tmsg)
+                    self.sendbuf = tmsg[sent:]
+                except BlockingIOError:
+                    self.sendbuf = tmsg
+            else:
+                self.sendbuf += tmsg
             self.last_sent = time.time()
 
     def got_message(self, message):