Measure scan progress from wallet birthday, not the fully scanned height

[nuttycom]

No description provided.

[zookozcash]

Not to go into SDK 2.0 Release Candidate, at least pending my imminent phone call with Kris.

[str4d]

The motivation for this change is that the current approach looks exponential, which gives the impression to users during recovery from seed or after long periods offline, of very little progress. Here are some snapshots from my mainnet node:

    chain_tip_height: BlockHeight(
        2217761,
    ),
    fully_scanned_height: BlockHeight(
        1373035,
    ),
    scan_progress: Some(
        Ratio {
            numerator: 48807,
            denominator: 72279746,
        },
    ),
     Synced: 0.068%
[...]
    chain_tip_height: BlockHeight(
        2217766,
    ),
    fully_scanned_height: BlockHeight(
        1722754,
    ),
    scan_progress: Some(
        Ratio {
            numerator: 48818,
            denominator: 71762038,
        },
    ),
     Synced: 0.068%
[...]
    chain_tip_height: BlockHeight(
        2217771,
    ),
    fully_scanned_height: BlockHeight(
        1732754,
    ),
    scan_progress: Some(
        Ratio {
            numerator: 48831,
            denominator: 67203704,
        },
    ),
     Synced: 0.073%
[...]
    chain_tip_height: BlockHeight(
        2217785,
    ),
    fully_scanned_height: BlockHeight(
        1772754,
    ),
    scan_progress: Some(
        Ratio {
            numerator: 48867,
            denominator: 39559642,
        },
    ),
     Synced: 0.124%

[zookozcash]

Update: Per phone call with Kris and str4d’s notes above in this PR, current implementation does not satisfy Criterion 3, because under the (probably common and important use case) that the user hasn’t let their wallet sync in months or years, it would show no visible progress for a long time (like at least minutes, potentially hours). So I’m reversing the above decision to exclude this change to progress measure for 2.0.

[str4d]

With this PR, I see the following progress state instead (from the same wallet sync state as the last snapshot above):

    chain_tip_height: BlockHeight(
        2217785,
    ),
    fully_scanned_height: BlockHeight(
        1772754,
    ),
    scan_progress: Some(
        Ratio {
            numerator: 33342966,
            denominator: 72853741,
        },
    ),
     Synced: 45.767%

[str4d]

ACK f56c5d8. Output appears to be more comprehensible for the catching-up-to-chain-tip use case.

[str4d]

Nit: now that start_height = birthday_height there's an off-by-one here, because the birthday height is one of the blocks that gets scanned, not the base block.

Non-blocking, the error in the output is minimal, and it isn't worth re-running CI.

[str4d]

Output 10k blocks later:

    chain_tip_height: BlockHeight(
        2217817,
    ),
    fully_scanned_height: BlockHeight(
        1782754,
    ),
    scan_progress: Some(
        Ratio {
            numerator: 38544695,
            denominator: 72853741,
        },
    ),
     Synced: 52.907%

[daira]

Post-hoc ACK