Add pending transactions cache

[pasqu4le]

closes #2429

Checklist for your 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 if necessary
• If I added/changed/removed ENV var, I should update the list of env vars in https://github.com/poanetwork/blockscout/blob/master/docs/env-variables.md to reflect changes in the table here https://poanetwork.github.io/blockscout/#/env-variables?id=blockscout-env-variables. I've set master in the Version column.
• If I add new indices into DB, I checked, that they don't redundant with PGHero or other tools

[coveralls]

Pull Request Test Coverage Report for Build e40a8662-8b35-4a83-a1cb-593e00cc2f9d

• 20 of 23 (86.96%) changed or added relevant lines in 8 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 78.518%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/explorer/lib/explorer/chain.ex	7	8	87.5%
apps/explorer/lib/explorer/chain/cache/pending_transactions.ex	0	2	0.0%

Totals
Change from base Build 5175c386-4c09-4591-9c3d-c7b43dde71a5:	0.02%
Covered Lines:	5278
Relevant Lines:	6722

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💛 - Coveralls