Instead of forcing all transactions inserted to use the same anchors, we
change the API to have unique anchors per transaction.
This allows for more flexibility in general. For example, use `Anchor`
implementations that contain the position in a block of a transaction.
The `insert_relevant_txs` test has also been changed to used
`KeychainTxOutIndex` so that index additions can be checked
(`SpkTxOutIndex` has no additions).
Additionally, generic bounds of some `IndexedTxGraph` list methods have
been fixed.
