Store bridged token price in the DB

[vbaranov]

Motivation

Bridge market cap updater can not get token price reliably from CoinGecko for the whole bridged tokens list

Changelog

Save exchange rate of token to the DB and gets old value from DB, if it is nil or 0 at CoinGecko scheduled update.

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 e3d0eb2109af1fbba2cab1b1789905b14f5cf82b-PR-3667

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.0%) to 64.8%

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Totals
Change from base Build c047c212f6ec595db2afe3fcea9da0ac2333103b:	1.0%
Covered Lines:	2106
Relevant Lines:	3250

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💛 - Coveralls