cosigner pool: use single thread to send messages

ServerProxy does not seem to be thread-safe.
For e.g. a 2of3 multisig wallet, which would send two messages,
one msg would get sent but the other might error out. See trace:

E | plugins.cosigner_pool.qt.Plugin | on_failure
Traceback (most recent call last):
  File "...\electrum\electrum\gui\qt\util.py", line 832, in run
    result = task.task()
  File "...\electrum\electrum\plugins\cosigner_pool\qt.py", line 199, in <lambda>
    task = lambda: server.put(_hash, message)
  File "...\Python38\lib\xmlrpc\client.py", line 1109, in __call__
    return self.__send(self.__name, args)
  File "...\Python38\lib\xmlrpc\client.py", line 1450, in __request
    response = self.__transport.request(
  File "...\Python38\lib\xmlrpc\client.py", line 1153, in request
    return self.single_request(host, handler, request_body, verbose)
  File "...\Python38\lib\xmlrpc\client.py", line 1165, in single_request
    http_conn = self.send_request(host, handler, request_body, verbose)
  File "...\Python38\lib\xmlrpc\client.py", line 1271, in send_request
    connection.putrequest("POST", handler, skip_accept_encoding=True)
  File "...\Python38\lib\http\client.py", line 1088, in putrequest
    raise CannotSendRequest(self.__state)
http.client.CannotSendRequest: Request-sent

electrum/plugins/cosigner_pool/qt.py

@@ -188,17 +188,27 @@ class Plugin(BasePlugin):
             except OSError: pass
             window.show_error(_("Failed to send transaction to cosigning pool") + ':\n' + repr(e))
 
+        buffer = []
+        some_window = None
+        # construct messages
         for window, xpub, K, _hash in self.cosigner_list:
             if not self.cosigner_can_sign(tx, xpub):
                 continue
-            # construct message
+            some_window = window
             raw_tx_bytes = tx.serialize_as_bytes()
             public_key = ecc.ECPubkey(K)
             message = public_key.encrypt_message(raw_tx_bytes).decode('ascii')
-            # send message
-            task = lambda: server.put(_hash, message)
-            msg = _('Sending transaction to cosigning pool...')
-            WaitingDialog(window, msg, task, on_success, on_failure)
+            buffer.append((_hash, message))
+        if not buffer:
+            return
+
+        # send messages
+        # note: we send all messages sequentially on the same thread
+        def send_messages_task():
+            for _hash, message in buffer:
+                server.put(_hash, message)
+        msg = _('Sending transaction to cosigning pool...')
+        WaitingDialog(some_window, msg, send_messages_task, on_success, on_failure)
 
     def on_receive(self, keyhash, message):
         self.logger.info(f"signal arrived for {keyhash}")