Pending transactions sanitizer process

[vbaranov]

Resolves #3665

Motivation

During the restart of the application, ongoing block import could result in the appearing of block with 0 transactions when actually it contains transactions.

Changelog

In order to eliminate the consequences of broken import, Indexer.PendingTransactionsSanitizer process is added which starts every 3 hours. It gets the list of pending transactions from the DB. It requests the archive node for each transaction hash from the list and if a transaction already included in the block, the process invalidates that block in Blokcsocut DB in order to re-fetch it. The process also removes pending transactions from the DB if they are dropped from the node.

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 c00dfbc4d936859c880d7cc3864bd3fa40758aa0-PR-3715

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 9 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-1.2%) to 64.393%

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Files with Coverage Reduction	New Missed Lines	%
test/block_scout_web/features/pages/chain_page.ex	1	90.0%
lib/block_scout_web/controllers/chain/market_history_chart_controller.ex	2	71.43%
lib/block_scout_web/checksum_address.ex	6	55.56%

Totals
Change from base Build ae03f9436b8044db7054f5766fcbfc12db0531c7:	-1.2%
Covered Lines:	2096
Relevant Lines:	3255

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💛 - Coveralls