Await completion of current requests before shutting down

[masih]

Reduce downtime for the clients by handling SIGINT gracefully; the service should:

• become "unready" to prohibit new requests arriving.
• await completion of ongoing requests.
• shut down once all is complete.

Once this feature is added, the termination timeout should be adjusted in K8S manifests to allow the container to wait enough for the requests to complete. Note the default timeout is 30 seconds which in the case of indexstar is plenty considering the current HTTP timeout configurations.