## Problem
Pinning a tenant by setting Pause scheduling policy doesn't work because
drain/fill code moves the tenant around during deploys.
Closes: https://github.com/neondatabase/neon/issues/9612
## Summary of changes
- In drain, only move a tenant if it is in Active or Essential mode
- In fill, only move a tenant if it is in Active mode.
The asymmetry is a bit annoying, but it faithfully respects the purposes
of the modes: Essential is meant to endeavor to keep the tenant
available, which means it needs to be drained but doesn't need to be
migrated during fills.
## Problem
Migrations of tenant shards with cold secondaries are holding up drains
in during production deployments.
## Summary of changes
If a secondary locations is lagging by more than 256MiB (configurable,
but that's the default), then skip cutting it over to the secondary as part of the node drain.
