Save block count in the DB when calculated in Cache module

[vbaranov]

Resolves #7032

Motivation

On application restart ETS cache is empty and blocks indexed ratio is based on estimated count from reltuples from pg_class query. Thus indexing banner may appear on every restart. We need to keep status of indexing in the DB as well.

Changelog

On every calculated from DB count of blocks from Explorer.Chain.Cache.Block module, save the counter value in last_fetched_counters table and use it, if ETS cache is empty.

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