Should atoms in toucan.hydrate flushed after defmodel?

When we change models, we may change `hydration-keys`, this change will not work unless we reload `toucan.hydrate` namespace. This looks inconvenient in development.

Since `automagic-batched-hydration-key->model` and `automagic-batched-hydration-keys` are delays, why not reload them after each time we call `defmodel`? maybe there's a better way for this?

Start by reading the toucan.hydrate namespace and the defmodel path, focusing on automagic-batched-hydration-key->model and automagic-batched-hydration-keys. Trace how hydration-keys changes are handled after defmodel; done means model changes work without manually reloading toucan.hydrate, or the repository documents a better approach.

Written by the indexing model from the issue text.