Address token balance on demand fetcher

[vbaranov]

Motivation

Address page in some cases shows stale tokens balance.

Changelog

As a patch (not a solid solution of the issue) on-demand token balance fetcher is introduced. With default stale balance window 24hrs.

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 9530e367232b19b82e7d89861a02ee7e6076724a-PR-3525

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 21 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.6%) to 66.677%

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Files with Coverage Reduction	New Missed Lines	%
lib/block_scout_web/checksum_address.ex	6	55.56%
lib/block_scout_web/controllers/address_token_balance_controller.ex	6	52.63%
lib/block_scout_web/notifier.ex	9	84.09%

Totals
Change from base Build c778962836e19ba99b01371d086b5af40db53d52:	0.6%
Covered Lines:	2069
Relevant Lines:	3103

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💛 - Coveralls