## Problem
Storage controller was observed to have unexpectedly large memory
consumption when loaded with many thousands of shards.
This was recently fixed:
- https://github.com/neondatabase/neon/pull/7493
...but we need a general test that the controller is well behaved with
thousands of shards.
Closes: https://github.com/neondatabase/neon/issues/7460
Closes: https://github.com/neondatabase/neon/issues/7463
## Summary of changes
- Add test test_storage_controller_many_tenants to exercise the system's
behaviour with a more substantial workload. This test measures memory
consumption and reproduces #7460 before the other changes in this PR.
- Tweak reconcile_all's return value to make it nonzero if it spawns no
reconcilers, but _would_ have spawned some reconcilers if they weren't
blocked by the reconcile concurrency limit. This makes the test's
reconcile_until_idle behave as expected (i.e. not complete until the
system is nice and calm).
- Fix an issue where tenant migrations would leave a spurious secondary
location when migrated to some location that was not already their
secondary (this was an existing low-impact bug that tripped up the
test's consistency checks).
On the test with 8000 shards, the resident memory per shard is about
20KiB. This is not really per-shard memory: the primary source of memory
growth is the number of concurrent network/db clients we create.
With 8000 shards, the test takes 125s to run on my workstation.
