postgres: use wait_for in disconnect

This is recommended by asyncpg [1].

1: https://github.com/MagicStack/asyncpg/blob/92c2d81256a1efd8cab12c0118d74ccd1c18131b/asyncpg/pool.py#L655-L656

databases/backends/postgres.py

@@ -1,3 +1,4 @@
+import asyncio
 import logging
 import typing
 from collections.abc import Mapping
@@ -66,7 +67,7 @@ async def connect(self) -> None:
 
     async def disconnect(self) -> None:
         assert self._pool is not None, "DatabaseBackend is not running"
-        await self._pool.close()
+        await asyncio.wait_for(self._pool.close(), timeout=30)
         self._pool = None
 
     def connection(self) -> "PostgresConnection":