[chain_redesign] Change behavior of try_get_chain_position

`TxGraph::try_get_chain_position` used to always exclude unconfirmed
transactions with last_seen value of 0. However, what is the point of
including a transaction in the graph if it cannot be part of the chain
history? Additionally, maybe sometimes we don't wish to use the
last_seen field at all.

The new behavior will consider unconfirmed transactions with last_seen
of 0.

crates/chain/tests/test_tx_graph.rs

@@ -717,10 +717,11 @@ fn test_chain_spends() {
         ObservedAs::Confirmed(&local_chain.get_block(95).expect("block expected"))
     );
 
-    // As long the unconfirmed tx isn't marked as seen, chain_spend will return None.
-    assert!(graph
-        .get_chain_spend(&local_chain, tip, OutPoint::new(tx_0.txid(), 1))
-        .is_none());
+    // Even if unconfirmed tx has a last_seen of 0, it can still be part of a chain spend.
+    assert_eq!(
+        graph.get_chain_spend(&local_chain, tip, OutPoint::new(tx_0.txid(), 1)),
+        Some((ObservedAs::Unconfirmed(0), tx_2.txid())),
+    );
 
     // Mark the unconfirmed as seen and check correct ObservedAs status is returned.
     let _ = graph.insert_seen_at(tx_2.txid(), 1234567);