refactor: call info once at the end of index writing

[dkharms]

Description

When we were building .index file for some reason we wanted to stored total amount of seq.ID (including seq.SystemID) in section of DocBlock offsets.

For sealing from Active fraction this is not a problem since we already have all necessary numbers in memory, but for compaction it is -- we do not know in advance total count of ids.

And, in my opinion, it is not correct to rely on Info while building index -- we can fully construct info only after index building.

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