Previously, `Persist{Async}With::persist` can be directly called as a
method on the type (i.e. `Wallet`). However, the `db: Db` (which is an
input) may not be initialized. We want a design which makes it harder
for the caller to make this mistake.
We change `Persist{Async}With::persist` to be an "associated function"
which takes two inputs: `db: &mut Db` and `changeset`. However, the
implementer cannot take directly from `Self` (as it's no longer an
input). So we introduce a new trait `Staged` which defines the staged
changeset type and a method that gives us a `&mut` of the staged
changes.
892b97d441