Simplify source piece & data handling during object retrieval

[teor2345]

This PR:

• removes an unnecessary re-allocation when retrieving object data
• removes the PieceIndex::source_ratio() method and its integer ratio assumption

As part of this change, more piece and segment index methods become const.

Code contributor checklist:

• I have read, understood and followed contributing guide

[nazar-pc]

The math is a bit tricky, bit appears to be correct. Though I would appreciate some quick unit tests. Will merge into the base branch and squash because I don't like the fix in the base branch and this one fixes my concerns completely.