Manage indexer memory limit

[vbaranov]

Resolves #5294

Motivation

Manage indexer memory limit via environment variable

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

[magnetto90]

@vbaranov I am getting the error "(Indexer.Fetcher.InternalTransaction) bound queue is at maximum" but memory limit has been removed from config? Can you please let me know how to change it in this new version?

Nevermind: the answer is here.