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>`.

example-crates/example_electrum/src/main.rs

@@ -278,20 +278,15 @@ fn main() -> anyhow::Result<()> {
 
     let missing_txids = {
         let graph = &*graph.lock().unwrap();
-        response
-            .missing_full_txs(graph.graph())
-            .cloned()
-            .collect::<Vec<_>>()
+        response.missing_full_txs(graph.graph())
     };
 
-    let new_txs = client
-        .batch_transaction_get(&missing_txids)
-        .context("fetching full transactions")?;
     let now = std::time::UNIX_EPOCH
         .elapsed()
         .expect("must get time")
         .as_secs();
-    let final_update = response.finalize(Some(now), new_txs);
+
+    let final_update = response.finalize(&client, Some(now), missing_txids)?;
 
     let db_changeset = {
         let mut chain = chain.lock().unwrap();