Allows us to store txs from the node. This currently doesn't add txs coming from the node, just adds the structs/methods to do so.
We only store the `txid`, `data_pos`, `undo_pos` and `time_first_seen` in the DB, everything else we can read from the block/undo files. We use the fact that coinbase txs don't have undo data, and undo data for txs never is at position 0, so we set `undo_pos = 0` for coinbase txs, and treat every entry with `undo_pos == 0` as a coinbase tx.
Incrementing `CURRENT_INDEXER_VERSION` is a technicality, as we add the column families to the DB, but don't add data from the node (yet), so using it later could result in an inconsistent state.
Depends on D13458.