connectd: Update connection list with new address

If we're already attempting to connect to a peer, we would ignore
new connection requests. This is problematic if your node has bad
connection details for the node -- you can't update it while inflight.

This patch appends new connection suggestions to the list of connections
to try.

Fixes #4154

tests/plugins/slow_start.py

@@ -0,0 +1,33 @@
+#!/usr/bin/env python3
+"""This plugin is used to check that updated connection hints work properly.
+
+"""
+from pyln.client import Plugin
+
+import socket
+import time
+
+plugin = Plugin()
+
+
+@plugin.async_method('waitconn')
+def wait_connection(request, plugin):
+    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+    sock.bind(('localhost', 0))
+    sock.listen(1)
+    print("listening for connections on port {}".format(sock.getsockname()[1]))
+
+    # We are a one and done socket connection!
+    conn, client_addr = sock.accept()
+    try:
+        print("connection from {}".format(client_addr))
+        time.sleep(3)
+
+    finally:
+        conn.close()
+
+    print("closing socket")
+    sock.close()
+
+
+plugin.run()