feat(scanner): Don't scan and log if we are below sapling height

[oxarbitrage]

Motivation

We want to log when the tip is below sapling and the scanner is activated.

Close #8041

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Solution

Get the current tip at each scanning loop iteration and compare it with sapling activation height. Do not scan if the tip is below the sapling activation height.

Testing

The scan_task_starts was modified to test this feature.

Review

Anyone can review.

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