chain: improvements to IndexedTxGraph and TxGraph APIs

For `IndexedTxGraph`:
- Remove `InsertTxItem` type (this is too complex).
    - `batch_insert_relevant` now uses a simple tuple `(&tx, anchors)`.
    - `batch_insert` is now also removed, as the same functionality can be
      done elsewhere.
- Add internal helper method `index_tx_graph_changeset` so we don't need
  to create a seprate `TxGraph` update in each method.
- `batch_insert_<relevant>_unconfirmed` no longer takes in an option of
  last_seen.
- `batch_insert_unconfirmed` no longer takes a reference of a
  transaction (since we apply all transactions anyway, so there is no
  need to clone).

For `TxGraph`:
- Add `batch_insert_unconfirmed` method.

crates/bdk/src/wallet/mod.rs

@@ -509,7 +509,7 @@ impl<D> Wallet<D> {
     where
         D: PersistBackend<ChangeSet>,
     {
-        let additions = self.indexed_graph.insert_txout(outpoint, &txout);
+        let additions = self.indexed_graph.insert_txout(outpoint, txout);
         self.persist.stage(ChangeSet::from(additions));
     }