Ignore unfamiliar messages by Explorer.Staking.ContractState module

[varasev]

Motivation

This small PR fixes a bug which led to permanent Staking DApp server side restarting due to unhandled :ssl_closed message. The bug relates to benoitc/hackney#464

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 b09b65b50c632dc026b7046f1f345160245eb850-PR-3620

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 7 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-2.1%) to 64.76%

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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/checksum_address.ex	6	55.56%

Totals
Change from base Build e94e3948c9e855dd231bd378a3efb00455d8eba2:	-2.1%
Covered Lines:	2095
Relevant Lines:	3235

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💛 - Coveralls