Manage BlockReward fetcher params

[vbaranov]

Motivation

Block reward fetcher batch size and concurrency are hardcoded.
Docs update blockscout/docs#120.

Changelog

New env vars to manage block reward fetcher params: INDEXER_BLOCK_REWARD_CONCURRENCY, INDEXER_BLOCK_REWARD_BATCH_SIZE

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