`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.
Instead of inserting anchors and seen_at timestamp in the same method,
we have three separate methods. This makes the API easier to understand
and makes `IndexedTxGraph` more consistent with the `TxGraph` API.
Other changes:
* The `async-https` feature of `bdk_esplora` is no longer default.
* Rename `ObservedAs` to `ChainPosition`.
* Set temporary MSRV to 1.60.0 to compile all workspace members will all
features.
This corresponds to `keychain::KeychainChangeSet` but for the redesigned
structures with `LocalChain`.
This structure is now used in `Wallet` as well as the examples.
