Enable graceful shutdown for the runner

[BuddhiWathsala]

Description:
AFAIK there is no proper mechanism to shutdown the Siddhi runner gracefully. I deployed the Siddhi runner in a K8s cluster. I had a requirement to enable zero downtime to the Siddhi runner deployment in K8s. I deployed the Siddhi app with HTTP source and route traffic using ingress NGINX controller.

Sometimes Siddhi runner terminates and another Siddhi runner deployment starting to consume HTTP requests. The terminating Siddhi runner has already established HTTP connections. But unfortunately, Siddhi runner shut down immediately without a response to the established connections. That leads to a 502 bad gateway in client-side.

In the termination, Siddhi runner gets a SIGTERM from K8s. At that time Siddhi runner should finish all the running threads and then starts to terminate.

Suggested Labels:
type/fix

Affected Product Version:
v5.1.0-m1

Steps to reproduce:

1. Deploy HTTP Siddhi app using Siddhi operator
2. Send continuous HTTP events
3. Update your Siddhi app and redeploy it in the K8s cluster
4. You will see some requests getting 502