Improve blocks handling in Staking DApp

[varasev]

Motivation

This improves blocks handling in the Staking DApp. Now the app can correctly handle possible block gaps and delays both on the client and server sides. Also, it should speed up the server side of the app as it now uses a separate thread for handling the :staking_update message (now it is handled by the StakingEventHandler instead of the RealtimeEventHandler).

These changes relate to #3400 and #3434.

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 9e5f9040a9adf7aa782904851bacf1ca53221009-PR-3443

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 211 unchanged lines in 6 files lost coverage.
• Overall coverage decreased (-0.9%) to 67.037%

---

Files with Coverage Reduction	New Missed Lines	%
lib/block_scout_web/application.ex	1	83.33%
test/block_scout_web/features/pages/chain_page.ex	1	90.0%
lib/block_scout_web/checksum_address.ex	6	55.56%
lib/block_scout_web/notifier.ex	12	85.06%
lib/block_scout_web/controllers/stakes_controller.ex	20	58.82%
lib/block_scout_web/channels/stakes_channel.ex	171	0%

Totals
Change from base Build 93198f2acc2070fef133d6b8a074d4dc687050b1:	-0.9%
Covered Lines:	2050
Relevant Lines:	3058

---

💛 - Coveralls

[vbaranov]

@varasev Looks good in overall, some minor changes are requested in order to increase readability.