## Problem
https://github.com/neondatabase/neon/pull/8588 implemented the mechanism
for storage controller
leadership transfers. However, there's no tests that exercise the
behaviour.
## Summary of changes
1. Teach `neon_local` how to handle multiple storage controller
instances. Each storage controller
instance gets its own subdirectory (`storage_controller_1, ...`).
`storage_controller start|stop` subcommands
have also been extended to optionally accept an instance id.
2. Add a storage controller proxy test fixture. It's a basic HTTP server
that forwards requests from pageserver
and test env to the currently configured storage controller.
3. Add a test which exercises storage controller leadership transfer.
4. Finally fix a couple bugs that the test surfaced
