Fix fetch_coin_balance query: coin balance delta

[vbaranov]

Motivation

Current query in fetch_coin_balance function causes performance issues.

Changelog

In order to address the performance of the query replace

SELECT a0."block_number", a0."value", a0."value_fetched_at", a0."inserted_at", a0."updated_at", a0."address_hash", value - coalesce(lag(value, 1) over (order by block_number), 0), b1."timestamp" FROM "address_coin_balances" AS a0 INNER JOIN "blocks" AS b1 ON a0."block_number" = b1."number" WHERE (a0."address_hash" = '\...') AND (a0."block_number" <= ...) ORDER BY a0."block_number" DESC LIMIT 1;

with

SELECT c."block_number", c."value", c."value_fetched_at", c."inserted_at", c."updated_at", c."address_hash", value - coalesce(lag(value, 1) over (order by block_number), 0) from (select a0."block_number", a0."value", a0."value_fetched_at", a0."inserted_at", a0."updated_at", a0."address_hash" FROM "address_coin_balances" AS a0  WHERE (a0."address_hash" = '\x...') AND (a0."block_number" <= ...) ORDER BY a0."block_number" DESC LIMIT 2) as c ORDER BY c."block_number" DESC LIMIT 1;

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 17908d004d8ade203aeaa5af8bf73c547593ad91-PR-3284

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+4.9%) to 75.836%

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Files with Coverage Reduction	New Missed Lines	%
lib/block_scout_web/controllers/chain/market_history_chart_controller.ex	2	71.43%

Totals
Change from base Build 716aa438993f01f20c73a46988cabc14d532cebf:	4.9%
Covered Lines:	1927
Relevant Lines:	2541

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💛 - Coveralls