Speedup query for getting verified smart-contract bytecode twin

[vbaranov]

Motivation

Some unverified contracts pages are not opened due to reaching timeout because of the slow query of checking verified contracts bytecode twins.

https://blockscout.com/poa/sokol/address/0x2D455d3bb60B37c88d25e7a8BB2B7f271317baB2/transactions

Changelog

Add storage of md5(contract_code) in smart_contracts in order to remove join with addresses table in the query of finding verified smart-contracts bytecode twins.

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