DISABLE_KNOWN_TOKENS env var

[vbaranov]

Motivation

There is no ability to switch off KnownTokens process

Changelog

Add DISABLE_KNOWN_TOKENS to manage KnownTokens fetcher

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 2d01ac51ebc933230d6389318ac934a9b1d990cc-PR-3404

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+12.5%) to 67.71%

---

Totals
Change from base Build fc6d477043406a49fb18e69add04a2b9d24afab8:	12.5%
Covered Lines:	2055
Relevant Lines:	3035

---

💛 - Coveralls