Sequential blocks broadcast on the main page

[vbaranov]

Motivation

Prevent broadcasting new block if previous one wasn't indexed yet. The fix is aimed to prevent situation like on this screenshot:

Changelog

• Store last broadcasted block number in in-memory cache (ETS).
• If block to be broadcasted has number > cached last broadcasted number + 1, broadcast function waits until previous blocks broadcasted by periodically requesting cached last broadcasted block number value every 0.5 sec.
• In case of 4 blocks gap, which is the maximum number of blocks on the main page, broadcast is allowed again.

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