Token balance on demand fetcher memory leak fix

[vbaranov]

Motivation

Memory leak in Token balance on demand fetcher

Changelog

Re-write Token balance on-demand fetcher as ordinary Module instead of Genserver process

Chore:

• Fix the issue when rendering of tokens dropdown at address page is called 6 times on the page: use checksummed address in the URI and ignore NaN balance in the state

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 9c183f10f32aa14be0a78d1181d0519e0224abae-PR-3735

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 7 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+2.01%) to 64.672%

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Files with Coverage Reduction	New Missed Lines	%
lib/block_scout_web/controllers/address_token_balance_controller.ex	7	58.33%

Totals
Change from base Build 73cde46bb59b20cfb4d23b49d34033e5a81362ab:	2.01%
Covered Lines:	2118
Relevant Lines:	3275

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💛 - Coveralls