server: use websocket's own broadcast method()

I hope this will reduce latency. Initial tests looked a bit better but
not great.

signalsrv/signalsrv.py

@@ -4,6 +4,7 @@ import json
 import logging
 import re
 
+from websockets.asyncio.connection import broadcast
 from websockets.asyncio.server import serve
 
 __VERSION__ = "0.0.1"
@@ -175,9 +176,7 @@ class Client:
                 raise
 
     async def _send_to_group(self, group, **kwargs):
-        async with asyncio.TaskGroup() as tg:
-            for member in group:
-                tg.create_task(member._send(ignore_exceptions=True, **kwargs))
+        broadcast([c.websocket for c in group], json.dumps(kwargs).encode() + b"\n")
 
     async def _send_error(self, msg: str):
         await self._send(type="error", message=msg)