Offset in blocks retrieval for average block time

[vbaranov]

Motivation

1. If non-consensus blocks are excluded in the calculation, the total amount of accounted blocks is <= 100. But it should be 100 for both cases: excluding/including of non-consensus blocks.
2. function refresh_timestamps gets the latest blocks but there could be reorgs and thus missing blocks in the DB that contaminates the average value.
   For instance, on Kovan it should be 4 sec:

Changelog

• add an offset of 100 blocks in refresh_timestamps function
• fixate limit of 100 target blocks for the case, when we exclude non-consensus blocks from calculation

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools

[coveralls]

Pull Request Test Coverage Report for Build 48f53e8c-d217-45fc-bb9f-41b5abdcec43

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.004%) to 75.018%

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Totals
Change from base Build 21dd49e8-0ec0-4b2b-a14e-88a97f0ee0c7:	-0.004%
Covered Lines:	5276
Relevant Lines:	7033

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💛 - Coveralls