zoekt: improve logs and resource usage when loading
Created by: keegancsmith
When an instance has a lot of shards it can take a while to start. This update includes two improvements to zoekt:
- shards: throttle loading in watcher https://gerrit-review.googlesource.com/c/zoekt/+/275902
- shards: a progress message every 10s when loading https://gerrit-review.googlesource.com/c/zoekt/+/275903
Note: a future change should make it possible to use k8s ready and health checks. For example we could then start exporting prometheus metrics on startup.