Add fast sync algorithm fixes

[nazar-pc]

This fixes a few issues I found in my 2 hour testing of #2763

It fixes frequent state sync download failures, improves block import performance and removes incorrect import of last block of second last segment that breaks invariants expected by archiver and violates specification.

I left a few TODOs I was lazy fixing immediately myself. But I did a few simplifications and removed some of the unnecessary changes.

Now code follows specification more closely except state import is not done atomically during import of the first block of last segment.

With these I have successfully synced Gemini 3h chain without any issues.

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