POSDAO refactoring: use 'getDelegatorPools' getter instead of 'getStakerPools' in Staking DApp

[varasev]

Motivation

This is a small refactoring PR that reflects the changes made in poanetwork/posdao-contracts@2994441. It is part of big refactoring in POSDAO contracts to add there an ability for changing pool's stakingAddress/miningAddress. We'll be making the changes step-by-step in small portions like this one.

Please, don't merge and publish Blockscout release with these changes until we upgrade the StakingAuRa contract to apply the changes. I'll let you know once that happens.

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 8c7de255fa36188bfe5c9be244de85ad14bcefb2-PR-3612

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 86 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+1.9%) to 64.8%

---

Files with Coverage Reduction	New Missed Lines	%
lib/block_scout_web/checksum_address.ex	6	55.56%
lib/block_scout_web/channels/stakes_channel.ex	80	0%

Totals
Change from base Build 7944cc535fddc0882043a717d85caec487db9bfb:	1.9%
Covered Lines:	2095
Relevant Lines:	3233

---

💛 - Coveralls

[varasev]

Now we can merge it to master