graceful shutdown

While https://github.com/owncloud/ocis/pull/10412 we allows services to deregister themselves, we still cannot shutdown the reva runtime properly. We effectivley exit(0) after three secs of receiving a SIGTERM or SIGINT. decomposedfs is never notified of the shutdown.

We should
- [ ] add a reva `runtime.Shutdown(ctx)` that allows a graceful shutdown like the http Server.
- [ ] return `/healthz` healty = false as soon as the service is starting shutting down. This cannot be done with a simple port check AFAICT. might need a channel or atomic boolean?

cc @fschade