Reduce max_concurrency of BlocksTransactionsMismatch

[pasqu4le]

Motivation

Due to the excessive pressure that this temporary fetcher puts on the CPU and the database, the max_concurrency has been reduced to just 1.

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

[coveralls]

Pull Request Test Coverage Report for Build 5136c09c-04fa-47cc-b2f2-129c209c7025

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 10 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.2%) to 80.674%

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Files with Coverage Reduction	New Missed Lines	%
apps/ethereum_jsonrpc/test/support/ethereum_jsonrpc/case/geth/http_websocket.ex	1	0.0%
apps/ethereum_jsonrpc/test/support/ethereum_jsonrpc/web_socket/case/geth.ex	4	0.0%
apps/ethereum_jsonrpc/lib/ethereum_jsonrpc/web_socket/web_socket_client.ex	5	77.5%

Totals
Change from base Build f0e7a751-cb83-49fd-ad0d-152211164560:	-0.2%
Covered Lines:	4763
Relevant Lines:	5904

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💛 - Coveralls