Check for zombie connections through last received payload

The previous code would check zombie connections depending on whether
HEARTBEAT_ACK was received. Unfortunately when there's exceeding
backpressure the connection can terminate since the HEARTBEAT_ACK is
buffered very far away despite it being there, just not received yet.

discord/gateway.py

@@ -119,12 +119,13 @@ class KeepAliveHandler(threading.Thread):
         self._stop_ev = threading.Event()
         self._last_ack = time.perf_counter()
         self._last_send = time.perf_counter()
+        self._last_recv = time.perf_counter()
         self.latency = float('inf')
         self.heartbeat_timeout = ws._max_heartbeat_timeout
 
     def run(self):
         while not self._stop_ev.wait(self.interval):
-            if self._last_ack + self.heartbeat_timeout < time.perf_counter():
+            if self._last_recv + self.heartbeat_timeout < time.perf_counter():
                 log.warning("Shard ID %s has stopped responding to the gateway. Closing and restarting.", self.shard_id)
                 coro = self.ws.close(4000)
                 f = asyncio.run_coroutine_threadsafe(coro, loop=self.ws.loop)
@@ -173,6 +174,9 @@ class KeepAliveHandler(threading.Thread):
     def stop(self):
         self._stop_ev.set()
 
+    def tick(self):
+        self._last_recv = time.perf_counter()
+
     def ack(self):
         ack_time = time.perf_counter()
         self._last_ack = ack_time
@@ -197,6 +201,7 @@ class VoiceKeepAliveHandler(KeepAliveHandler):
     def ack(self):
         ack_time = time.perf_counter()
         self._last_ack = ack_time
+        self._last_recv = ack_time
         self.latency = ack_time - self._last_send
         self.recent_ack_latencies.append(self.latency)
 
@@ -429,6 +434,9 @@ class DiscordWebSocket:
         if seq is not None:
             self.sequence = seq
 
+        if self._keep_alive:
+            self._keep_alive.tick()
+
         if op != self.DISPATCH:
             if op == self.RECONNECT:
                 # "reconnect" can only be handled by the Client