chain: improvements to `IndexedTxGraph` and `TxGraph` APIs

chain: improvements to `IndexedTxGraph` and `TxGraph` APIs

For `IndexedTxGraph`:
- Remove `InsertTxItem` type (this is too complex).
    - `batch_insert_relevant` now uses a simple tuple `(&tx, anchors)`.
    - `batch_insert` is now also removed, as the same functionality can be
      done elsewhere.
- Add internal helper method `index_tx_graph_changeset` so we don't need
  to create a seprate `TxGraph` update in each method.
- `batch_insert_<relevant>_unconfirmed` no longer takes in an option of
  last_seen.
- `batch_insert_unconfirmed` no longer takes a reference of a
  transaction (since we apply all transactions anyway, so there is no
  need to clone).

For `TxGraph`:
- Add `batch_insert_unconfirmed` method.