Get last value of fetched coinsupply API endpoint from DB if cache is empty

[vbaranov]

Motivation

coinsupply API endpoint returns 0 supply for sometime after restart of the application because it retrieved from ETS (in-memory cache), which is cleared on application restart.

Changelog

In order to provide uninterrupted service for coinsupply API endpoint, it is suggested to store last fetched value on cache update for this counter in the DB. And if cache is empty (for instance, on application restart), get that value from DB.

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 8887eef5-439a-404a-94a3-2148153cace5

• 0 of 11 (0.0%) changed or added relevant lines in 4 files are covered.
• 8 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.2%) to 74.912%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/explorer/lib/explorer/chain/cache/address_sum_minus_burnt.ex	0	2	0.0%
apps/explorer/lib/explorer/counters/last_fetched_counter.ex	0	2	0.0%
apps/block_scout_web/lib/block_scout_web/controllers/api/rpc/stats_controller.ex	0	3	0.0%
apps/explorer/lib/explorer/chain.ex	0	4	0.0%

Files with Coverage Reduction	New Missed Lines	%
apps/block_scout_web/lib/block_scout_web/controllers/api/rpc/stats_controller.ex	1	60.0%
apps/explorer/lib/explorer/chain/cache/address_sum_minus_burnt.ex	1	0%
apps/block_scout_web/lib/block_scout_web/checksum_address.ex	6	55.56%

Totals
Change from base Build 778b42fc-02a4-4875-9253-a23897f27ebd:	-0.2%
Covered Lines:	5718
Relevant Lines:	7633

---

💛 - Coveralls