Add healthz endpoint (readiness and liveness checks) to all services

Created by: ggilmore

All of our microservices should have lightweight readiness and liveness checks. These help k8s know when to restart a container.

The following services are missing these checks:

• github-proxy: https://github.com/sourcegraph/deploy-sourcegraph/blob/master/base/github-proxy/github-proxy.Deployment.yaml
• query-runner: https://github.com/sourcegraph/deploy-sourcegraph/blob/master/base/query-runner/query-runner.Deployment.yaml
• repo-updater: https://github.com/sourcegraph/deploy-sourcegraph/blob/master/base/repo-updater/repo-updater.Deployment.yaml
• synctect-server: https://github.com/sourcegraph/deploy-sourcegraph/blob/master/base/syntect-server/syntect-server.Deployment.yaml