Exclude nonconsensus blocks from avg block time calculation by default

[vbaranov]

Motivation

Excluding of nonconsensus blocks from avg block time calculation was introduced here #2266. But the default option was inclusion of such blocks whereas the most popular option is to calculate only consensus blocks.

Changelog

• Change the default option of avg block time calculation - exclude nonconsensus blocks by default
• Add UNCLES_IN_AVERAGE_BLOCK_TIME to docs. It was missing.

Checklist for your 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 if necessary
• If I added/changed/removed ENV var, I should update the list of env vars in https://github.com/poanetwork/blockscout/blob/master/docs/env-variables.md to reflect changes in the table here https://poanetwork.github.io/blockscout/#/env-variables?id=blockscout-env-variables. I've set master in the Version column.
• If I add new indices into DB, I checked, that they don't redundant with PGHero or other tools

[coveralls]

Pull Request Test Coverage Report for Build b080c9bc-f530-46f5-9d9f-773315725c28

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 78.419%

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Totals
Change from base Build af1d77a6-23e4-4bb7-be63-b6ed26bc56b7:	0.0%
Covered Lines:	5189
Relevant Lines:	6617

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💛 - Coveralls