Refactor runtime config

[vbaranov]

Motivation

A lot of patterns in runtime config

Changelog

Define common functions to parse integer, time and boolean env variables from runtime config.
Docs update in blockscout/docs#128

Chore: Rename all "helpers" modules to singular ("helper").

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

[sl1depengwyn]

Can this be replaced with parse_time_env_var?

Also in some places we explicitly set nil as default value for System.get_env but in majority of cases we leave it as a default argument, I suggest to stick to one solution for consistency and either remove explicit nils or add them everywhere