Provide a suggested checkpoint if rollback_to slot doesn't exist

[KtorZ]

Discussed in #90

^{Originally posted by Quantumplation December 10, 2022}

Describe your idea, in simple words.

Suppose I want to track a new wallet, and I know the slot where I first sent it funds;

I don't want to provide that slot as the rollback point, because that will exclude the initial funds that were sent to the wallet.

So, I take the slot number (something like 79136895), and truncate it to a nice whole number (something like 79136000). This fails.

My suggestion is to either:

• provide an alternative payload that allows me to specify ancestor_to which uses the checkpoint to find the most recent ancestor
• update the hint message that is returned to describe this, and include a suggested slot, such as

{
  "hint":"The provided point (or slot) is unknown. Please provide an existing point from the past.  Either a block wasn't produced in that slot, or it is very recent. Note that if you provide a very recent point (e.g. 1-3 blocks old), it can happen that the point disappear should a rollback happen between the moment you looked it up and the moment you make this query.",
  "suggested_checkpoint": "79135995"
}

Why is it a good idea?

The first proposal would reduce the needed API calls by one, and save a round trip.
The second proposal would at least make the workflow, when doing things by hand, more obvious.

Are you willing to work on it yourself?

Yes, but as always, I doubt I'd have the time unless someone were to provide some hints at source files to look at. 😅