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>`.

crates/bdk/src/wallet/mod.rs

@@ -97,7 +97,8 @@ pub struct Wallet<D = ()> {
 pub type Update = LocalUpdate<KeychainKind, ConfirmationTimeAnchor>;
 
 // /// The changeset produced internally by applying an update.
-pub(crate) type ChangeSet = LocalChangeSet<KeychainKind, ConfirmationTimeAnchor>;
+pub type ChangeSet = LocalChangeSet<KeychainKind, ConfirmationTimeAnchor>;
+
 /// The address index selection strategy to use to derived an address from the wallet's external
 /// descriptor. See [`Wallet::get_address`]. If you're unsure which one to use use `WalletIndex::New`.
 #[derive(Debug)]