fix(backend/postgres): allow concurrent pubs

This fix adds a lock (asyncio.Event based) to avoid
`asyncpg.exceptions._base.InterfaceError: cannot perform operation: another operation is in progress`

fixes #22

broadcaster/_backends/postgres.py

@@ -13,6 +13,8 @@ def __init__(self, url: str):
 
     async def connect(self) -> None:
         self._conn = await asyncpg.connect(self._url)
+        self._event = asyncio.Event()
+        self._event.set()
         self._listen_queue: asyncio.Queue = asyncio.Queue()
 
     async def disconnect(self) -> None:
@@ -25,7 +27,12 @@ async def unsubscribe(self, channel: str) -> None:
         await self._conn.remove_listener(channel, self._listener)
 
     async def publish(self, channel: str, message: str) -> None:
-        await self._conn.execute("SELECT pg_notify($1, $2);", channel, message)
+        await self._event.wait()
+        self._event.clear()
+        try:
+            await self._conn.execute("SELECT pg_notify($1, $2);", channel, message)
+        finally:
+            self._event.set()
 
     def _listener(self, *args: Any) -> None:
         connection, pid, channel, payload = args