This adds zonal high availability to the product. Multiple Coder servers started in the same zone will mesh and coordinate automatically.

I tested this manually by deploying to Kubernetes with 50 replicas and spawning 50 workspaces. It all works!

Some cool extras:

1. If TLS is used, we reuse this certificate to encrypt traffic between Coder pods for meshing.
2. If replicas cannot dial each other, an error will be returned on the API object. This will be used in the future to make a nice UI!
3. Database latency is exposed as a stat on replicas, and this will be expanded in the future to other stats for monitoring.

Closes #3227