## Problem
- https://github.com/neondatabase/neon/issues/7355
The optimize_secondary function calls schedule_shard to check for
improvements, but if there are exactly the same number of nodes as there
are replicas of the shard, it emits some scary looking logs about no
nodes being elegible.
Closes https://github.com/neondatabase/neon/issues/7355
## Summary of changes
- Add a mode to SchedulingContext that controls logging: this should be
useful in future any time we add a log to the scheduling path, to avoid
it becoming a source of spam when the scheduler is called during
optimization.
No functional changes, this is a comments/naming PR.
While merging sharding changes, some cleanup of the shard.rs types was
deferred.
In this PR:
- Rename `is_zero` to `is_shard_zero` to make clear that this method
doesn't literally mean that the entire object is zeros, just that it
refers to the 0th shard in a tenant.
- Pull definitions of types to the top of shard.rs and add a big comment
giving an overview of which type is for what.
Closes: https://github.com/neondatabase/neon/issues/6072
The binary etc were renamed some time ago, but the path in the source
tree remained "attachment_service" to avoid disruption to ongoing PRs.
There aren't any big PRs out right now, so it's a good time to cut over.
- Rename `attachment_service` to `storage_controller`
- Move it to the top level for symmetry with `storage_broker` & to avoid
mixing the non-prod neon_local stuff (`control_plane/`) with the storage
controller which is a production component.
