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/bitcoind_rpc/tests/test_emitter.rs

@@ -392,7 +392,6 @@ fn tx_can_become_unconfirmed_after_reorg() -> anyhow::Result<()> {
             get_balance(&recv_chain, &recv_graph)?,
             Balance {
                 confirmed: SEND_AMOUNT * (ADDITIONAL_COUNT - reorg_count) as u64,
-                trusted_pending: SEND_AMOUNT * reorg_count as u64,
                 ..Balance::default()
             },
             "reorg_count: {}",