[Core][Qt] Rewind block index to last checkpoint functionality

[Liquid369]

This pull request supersedes #2298

This feature is a good addition because in cases of late updates, or a short fork a user can start from the latest hardcoded checkpoints instead of re-syncing from scratch.

Cleaned up a little and added the missing functionality that was remaining.

This PR now:

• Allows starting with -rewindblockindex param
• Added GUI section Rewind Blockchain inside "Wallet Repair" tab
• Functional test
• New Checkpoint

Potential TODO:

• More checkpoints?

Some work was done by PeterL73

[PeterL73]

Could we automatically detect if a rewind is needed?
If there are checkpoints between the tip and the last (known) checkpoint, we are not on the right chain.
Maybe something like this:

// If there are checkpoints between prevCheckPoint and chainActive.Tip
// This is not the right chain and a rewind to prevCheckPoint is needed
bool bNeedRewind = fJustCheck;
if (Checkpoints::fEnabled){
    const MapCheckpoints& checkpoints = *Params().Checkpoints().mapCheckpoints;

    for (const auto& i : reverse_iterate(checkpoints)) {
        if (i.first > checkPointHeight && i.first <= nHeight) {
            bNeedRewind = true;
            break;
        }
    }
}

if(!bNeedRewind)
    return true;

[panleone]

If there are checkpoints between the tip and the last (known) checkpoint, we are not on the right chain. and this is right,
but if there are not checkpoints between the tip and the last (known) checkpoint it does not necessarily mean we are on the right chain

[Liquid369]

I do not see a possibility in automatically detecting a rewind.

In such we have two cases:
1.
There is a checkpoint known and in the future, but we are behind said checkpoint, we would then simply sync to it.
2.
There is not a checkpoint and between the tip and the last known, to which we should be fine.

IE: Checkpoint block is 2150000, we are at block 2150010, if there was any in-between, that user would already have synced those
Checkpoint is block 2150005 and 2150000, while we are at block 2150010, there's no way we would be at 2150010 with a prevCheckpoint of 2150000 not 2150005 and on the same wallet version.

[panleone]

Successfully tested on mainnet and code looks really good! Just a couple of comments:

[panleone]

This checkpoint is useless

[Liquid369]

I agree! I should have prefaced thats for testing since the last checkpoint was a big further back and would have been a faster sync

[panleone]

hence re-update those with last v5.5 checkpoint

[Liquid369]

Agreed, I will rebase and we just make sure to update more checkpoints more frequently