Reduce RPC requests and DB changes by Staking DApp

[varasev]

Motivation

Staking DApp requests information about pools and delegators (and rewrites the corresponding DB tables) on every block even when there are no changes in StakingAuRa and ValidatorSetAuRa contracts. This PR makes DApp only do that when some changes occurred. For that purpose, it uses the lastChangeBlock field from the contracts introduced in poanetwork/posdao-contracts@ef9ccff

Also, this PR comments out the warning added in #3576 (but doesn't remove it completely).

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 fed392f6b441d8daf91f34d422bcf1379cf52757-PR-3583

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.9%) to 66.742%

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Files with Coverage Reduction	New Missed Lines	%
test/block_scout_web/features/pages/chain_page.ex	1	90.0%

Totals
Change from base Build 81d3a8162fe381a96af110ace9df582f00f7d03b:	0.9%
Covered Lines:	2077
Relevant Lines:	3112

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💛 - Coveralls