gravel: gstate: close kvstore on shutdown

Without this, KV's cluster connect thread hangs around forever if you
try to stop the aquarium service (well, it hangs around for a minute
and a half until systemd gets sick of asking nicely and just kills the
process - still, better to do this cleanly).

Signed-off-by: Tim Serong <tserong@suse.com>

src/gravel/controllers/gstate.py

@@ -173,6 +173,7 @@ async def start(self) -> None:
 
     async def shutdown(self) -> None:
         self._is_shutting_down = True
+        await self._kvstore.close()
         logger.info("shutdown!")
         await self.tick_task