[Add] ping_interval

增加了一个变量，用来控制websocket_client中ping的时间间隔。

vnpy/api/websocket/websocket_client.py

@@ -50,9 +50,10 @@ class WebsocketClient(object):
         self._last_sent_text = None
         self._last_received_text = None
 
-    def init(self, host: str, proxy_host: str = "", proxy_port: int = 0):
+    def init(self, host: str, proxy_host: str = "", proxy_port: int = 0, ping_interval: int = 60):
         """"""
         self.host = host
+        self.ping_interval = ping_interval  # seconds
 
         if proxy_host and proxy_port:
             self.proxy_host = proxy_host
@@ -202,7 +203,7 @@ class WebsocketClient(object):
                 et, ev, tb = sys.exc_info()
                 self.on_error(et, ev, tb)
                 self._reconnect()
-            for i in range(60):
+            for i in range(self.ping_interval):
                 if not self._active:
                     break
                 sleep(1)