fix: reserve size to return chunks with po greater than and equal to storage radius

[istae]

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• [ ] My change requires a documentation update, and I have done it.
• [ ] My change requires updating the Open API specification and/or changing its version, and I've done it.
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Description

When the depthmonitor changes the storage radius, this does not automatically alter the radius that the localstore keeps in it's reserve, so the ReserveSize does not return an accurate number. With this PR, we iterate on the pull index to manually count all chunks whose PO is greater than or equal to the storage radius.

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