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.
