feat!: Improve spk-based syncing flow

feat!: Improve spk-based syncing flow

* Change `TxUpdate::seen_ats` to be a `HashSet<(Txid, u64)>` so we can
  introduce multiple timestamps per tx. This is useful to introduce both
  `first_seen` and `last_seen` timestamps to `TxGraph`. This is also a
  better API for chain-sources as they can just insert timestamps into
  the field without checking previous values.
* Change sync/full-scan flow to have the request structure introduce the
  sync time instead of introducing the timestamp when apply the
  `TxUpdate`. This simplifies the `apply_update{_at}` logic and makes
  `evicted_at` easier to reason about (in the future).