`bdk_electrum` API improvements and simplifications

`bdk_electrum` API improvements and simplifications

* `ElectrumUpdate::missing_full_txs` now returns a `Vec<Txid>` so we
  don't keep a reference to the passed-in `graph`.

* `ElectrumUpdate::finalize*` methods now takes in `missing` txids
  instead of `full_txs`. `Client::batch_transaction_get` is called
within the methods.

Other changes:

* `wallet::ChangeSet` is now made public externally. This is required as
  a wallet db should implement `PersistBackend<wallet::ChangeSet>`.