Avoid replotting the same sector more than once

[nazar-pc]

If replotting was taking too long it was possible for new segment to be archived and replotting to be triggered again for the same sector, resulting in mishmash of contents written to disk.

While it was nice to only insert entry into sectors_being_modified due to it being still available for farming in the meantime, the value of this is limited even on devnet, let alone on bigger networks with more space pledged.

The fix is to insert an entry before even starting plotting and removing at the very end.

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