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.

example-crates/example_bitcoind_rpc_polling/src/main.rs

@@ -212,8 +212,7 @@ fn main() -> anyhow::Result<()> {
 
             // mempool
             let mempool_txs = emitter.mempool()?;
-            let graph_changeset = graph
-                .batch_insert_unconfirmed(mempool_txs.iter().map(|(tx, time)| (tx, Some(*time))));
+            let graph_changeset = graph.batch_insert_unconfirmed(mempool_txs);
             db.stage((local_chain::ChangeSet::default(), graph_changeset));
 
             // commit one last time!
@@ -291,7 +290,7 @@ fn main() -> anyhow::Result<()> {
                     }
                     Emission::Mempool(mempool_txs) => {
                         let graph_changeset = graph.batch_insert_relevant_unconfirmed(
-                            mempool_txs.iter().map(|(tx, time)| (tx, Some(*time))),
+                            mempool_txs.iter().map(|(tx, time)| (tx, *time)),
                         );
                         (local_chain::ChangeSet::default(), graph_changeset)
                     }