Fix performance of coin supply API endpoints

[vbaranov]

Fixes #3131

Motivation

Queries for coin supply endpoints are slow. For instance:

select SUM(a0.fetched_coin_balance) from addresses a0;

Changelog

However, addresses table has index "addresses_fetched_coin_balance_index" btree (fetched_coin_balance) on fetched_coin_balance column which excludes empty or zero balances and it is not used by the query. We can modify query adding where fetched_coin_balance > 0 clause to speed-up it without changing the result of the query.

core=> explain analyze select SUM(a0.fetched_coin_balance) from addresses a0 where fetched_coin_balance > 0;
                                                                                QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=3011.43..3011.44 rows=1 width=32) (actual time=48.242..48.243 rows=1 loops=1)
   ->  Index Only Scan using addresses_fetched_coin_balance_index on addresses a0  (cost=0.44..2807.17 rows=81704 width=3) (actual time=0.027..27.634 rows=73969 loops=1)
         Index Cond: (fetched_coin_balance > '0'::numeric)
         Heap Fetches: 2834
 Planning time: 0.244 ms
 Execution time: 48.284 ms
(6 rows)

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 8d6cc613-5a8b-4f2e-b77a-8cfa15c561d9

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 75.151%

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Totals
Change from base Build 46f5ee1f-79aa-4653-acc8-11d470595cdd:	0.1%
Covered Lines:	5728
Relevant Lines:	7622

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💛 - Coveralls