Removes duplicate entries from Indexer.Fetcher.UncleBlock

[pasqu4le]

Resolves #2154
Resolves #2048

Motivation

Sometimes the same uncle gets fetched more than once in the same batch, causing the updating query to fail. As a solution, the Fetcher input entries are deduplicated.

Bug Fixes

I was not able to pinpoint the cause of the duplicate, but my best guess is that it is related by the asynchronicity of the fetching that causes Explorer.Chain.stream_unfetched_uncles to refetch an uncle before it's imported and its uncle_fetched_at gets set.

Given that the results from processing multiple times the same entry are the same and that there are no particular constraints on_conflictfor SecondDegreeRelations, the solution consists in simply eliminating the duplicates from the fetcher entries.

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 2e18da10-7207-42cc-a79c-a1532e526fbf

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 80.866%

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Files with Coverage Reduction	New Missed Lines	%
apps/indexer/lib/indexer/fetcher/internal_transaction.ex	2	58.62%

Totals
Change from base Build d1457540-d67d-4902-a1e3-627de9ba07e1:	-0.03%
Covered Lines:	4932
Relevant Lines:	6099

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💛 - Coveralls