fix(tx_graph)!: Change tx_last_seen to Option<u64>

Also fixup `test_list_owned_txouts` to check that the right
outputs, utxos, and balance are returned at different local
chain heights.

This fixes an issue where unbroadcast and otherwise non-canonical
transactions were returned from methods `list_chain_txs` and
`Wallet::transactions` because every tx inserted had a last_seen
of 0 making it appear unconfirmed.

Note this commit changes the way `Balance` is represented due to
new logic in `try_get_chain_position` that no longer considers
txs with non-canonical anchors. Before this change, a tx anchored
to a block that is reorged out had a permanent effect on the
pending balance, and now only txs with a last_seen time or an
anchor confirmed in the best chain will return a `ChainPosition`.

crates/chain/tests/common/tx_template.rs

@@ -131,9 +131,7 @@ pub fn init_graph<'a, A: Anchor + Clone + 'a>(
         for anchor in tx_tmp.anchors.iter() {
             let _ = graph.insert_anchor(tx.compute_txid(), anchor.clone());
         }
-        if let Some(seen_at) = tx_tmp.last_seen {
-            let _ = graph.insert_seen_at(tx.compute_txid(), seen_at);
-        }
+        let _ = graph.insert_seen_at(tx.compute_txid(), tx_tmp.last_seen.unwrap_or(0));
     }
     (graph, spk_index, tx_ids)
 }