feat!: change load_from_persistence to return an option

`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.

example-crates/example_cli/src/lib.rs

@@ -687,7 +687,7 @@ where
         Err(err) => return Err(anyhow::anyhow!("failed to init db backend: {:?}", err)),
     };
 
-    let init_changeset = db_backend.load_from_persistence()?;
+    let init_changeset = db_backend.load_from_persistence()?.unwrap_or_default();
 
     Ok((
         args,