[bdk_chain_redesign] Fix `tx_graph::Additions::append` logic

[bdk_chain_redesign] Fix `tx_graph::Additions::append` logic

* `Additions` now implements `Append` and uses `Append` to implement
  `append()`.
* `append()` logic enforces that `last_seen` values should only
  increase.
* Test written for `append()` with `last_seen` behaviour.