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

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`.