This test simulates a situation where the last write to the db is short.
Aggregating the changeset after reopening the file should return an
error (which includes a partially-aggregated changeset) containing an
aggregation of changesets that were fully written.
At this point, the test re-writes the final changeset (and this time it
successfully writes in full).
The file should be recoverable with all changesets, including the last
one.
`PersistBackend::is_empty` is removed. Instead, `load_from_persistence`
returns an option of the changeset. `None` means persistence is empty.
This is a better API than a separate method. We can now differentiate
between a persisted single changeset and nothing persisted.
`Store::aggregate_changeset` is refactored to return a `Result` instead
of a tuple. A new error type (`AggregateChangesetsError`) is introduced
to include the partially-aggregated changeset in the error. This is a
more idiomatic API.
This is a more generic version of `keychain::persist::*` structures.
Additional changes:
* The `Append` trait has a new method `is_empty`.
* Introduce `Store` structure for `bdk_file_store`.
