There are a number of improvements that can be done, but it is in a
decent state to be usable.
Possible improvements:
* Remove requirement to retry obtaining ALL data after reorg is
detected. Transactions can be anchored to a lower block (not block
tip), and an `assume_final_depth` value can be used.
* The logic to finalize an update with confirmation time can be improved
during reorgs to not require returning an error.
As mentioned by @LLFourn:
1. We have a "sparse chain" from which there is a subset of txids M that are missing from graph.
2. There is also another subset C that are in the graph but their positions have changed.
3. We used the Cow to avoid copying/duplicating in memory transactions in subset C and M
Instead in inflate_update we could remove transactions in subset M and just clone data in subset C (which is usually tiny).
