Take into account FIRST_BLOCK for block rewards fetching

[vbaranov]

Motivation

blockscout     | 2020-05-27T07:53:08.769 application=indexer fetcher=block_reward count=10 error_count=10 [error] failed to fetch: @�: (-32603) Internal error
blockscout     | @�: (-32603) Internal error
blockscout     | @�: (-32603) Internal error
blockscout     | @�: (-32603) Internal error
blockscout     | @�: (-32603) Internal error
blockscout     | @�: (-32603) Internal error
blockscout     | @�: (-32603) Internal error
blockscout     | @�: (-32603) Internal error
blockscout     | @�: (-32603) Internal error
blockscout     | @�: (-32603) Internal error

Changelog

Validate blocks which send to trace_block JSON RPC method. The numbers should be greater than FIRST_BLOCK param.

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 017d1d20-defc-4659-b7ce-62566cfe689e

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 75.089%

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Files with Coverage Reduction	New Missed Lines	%
apps/indexer/lib/indexer/block/catchup/bound_interval_supervisor.ex	3	85.19%

Totals
Change from base Build b26d515d-8cb1-47ca-a4c6-3a8086bd0b4a:	-0.02%
Covered Lines:	5721
Relevant Lines:	7619

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💛 - Coveralls