graceful shutdown

[butonic]

While #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

[jvillafanez]

There was some work done in #9048 , but got stuck at the same point with the reva services.
I'm not sure how relevant that PR is with the latest changes.