zoekt: add grafana dashboards for job queueing delays

Created by: ggilmore

Follow up to https://github.com/sourcegraph/zoekt/pull/331

This PR adds new Grafana dashboards that surface zoket-indexserver's job queuing delay. The hope is that we can use these to help us debug strange indexing delays that we can see on sourcegraph.com.

Test plan

The existing unit tests, and manually testing against k8s.sgdev.org