Run token balance on-demand fetcher in parallel

[vbaranov]

Finalization of #3525

Motivation

Changelog

Run token balance on-demand fetcher in parallel non-blocking process via Task. start_link

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 81c16f5859d0f3454b8366d7facece793984b530-PR-3533

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.3%) to 66.86%

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Files with Coverage Reduction	New Missed Lines	%
lib/block_scout_web/controllers/address_token_balance_controller.ex	5	55.0%

Totals
Change from base Build 84d9d0923247315a8e2389ba63ba2be39ca33d2e:	0.3%
Covered Lines:	2076
Relevant Lines:	3105

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💛 - Coveralls