network: make MAX_INCOMING_MSG_SIZE configurable from config

requested in https://github.com/spesmilo/electrum/issues/4315#issuecomment-698730778

electrum/interface.py

@@ -126,13 +126,13 @@ def assert_list_or_tuple(val: Any) -> None:
 
 class NotificationSession(RPCSession):
 
-    def __init__(self, *args, **kwargs):
+    def __init__(self, *args, interface: 'Interface', **kwargs):
         super(NotificationSession, self).__init__(*args, **kwargs)
         self.subscriptions = defaultdict(list)
         self.cache = {}
         self.default_timeout = NetworkTimeout.Generic.NORMAL
         self._msg_counter = itertools.count(start=1)
-        self.interface = None  # type: Optional[Interface]
+        self.interface = interface  # type: Interface
         self.cost_hard_limit = 0  # disable aiorpcx resource limits
 
     async def handle_request(self, request):
@@ -209,7 +209,9 @@ class NotificationSession(RPCSession):
 
     def default_framer(self):
         # overridden so that max_size can be customized
-        return NewlineFramer(max_size=MAX_INCOMING_MSG_SIZE)
+        max_size = int(self.interface.network.config.get('network_max_incoming_msg_size',
+                                                         MAX_INCOMING_MSG_SIZE))
+        return NewlineFramer(max_size=max_size)
 
 
 class NetworkException(Exception): pass
@@ -605,7 +607,8 @@ class Interface(Logger):
         return self.network.default_server == self.server
 
     async def open_session(self, sslc, exit_early=False):
-        async with _RSClient(session_factory=NotificationSession,
+        session_factory = lambda *args, iface=self, **kwargs: NotificationSession(*args, **kwargs, interface=iface)
+        async with _RSClient(session_factory=session_factory,
                              host=self.host, port=self.port,
                              ssl=sslc, proxy=self.proxy) as session:
             self.session = session  # type: NotificationSession