services/horizon: Remove all references to Redis

[2opremio]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

Fixes #2395

What

Remove all Redis references.

Why

It wasn't used, it was only adding complexity and causing test failures.

Known limitations

N/A

[leighmcculloch]

We used to add a specially formatted comment following the pattern outlined in issue #833 so that CI picks up on them and helps us remove them when the time comes. @stellar/horizon-committers I can't find any details in our CONTRIBUTING.md or DEVELOPING.md documents that outline how to do this. Have we abandoned that practice?

We still have the CI check for this (below), so I think we're still wanting to do it and probably need some better docs to encourage us to use it. I know I've probably not done this properly recently too 😬.

go/.circleci/config.yml

Lines 41 to 52 in bab6abc

	# check_deprecations ensures a release is actually removing deprecated fields
	# that were supposed to be discontinued in said release.
	check_deprecations:
	steps:
	- run:
	name: Run deprecation tests when on a tagged commit
	command: |
	if [ "$CIRCLE_TAG" != "" ]; then
	# Negate the result so process exits with 1 if anything found
	echo "Searching for \"action needed\" tags..."
	! egrep -irn -A 1 --include=*.go "Action.+needed.+in.+release:.+$CIRCLE_TAG" ./
	fi

[ire-and-curses]

Good catch. The approach isn't abandoned. It's documented in our internal devel guide.

Mark deprecated public API fields using deprecation tag:
// Deprecated - remove in: horizon-v[version]

We should do a pass over this and update DEVELOPING.md with things relevant to all contributors.

@2opremio can you update the deprecation comments to match the syntax quoted above?

[ire-and-curses]

#2412 for the doc updates

[2opremio]

sure thing

[leighmcculloch]

I notice that this change was made only for the next Horizon release branch. Is there a reason this isn't on master? I may be making changes to the CircleCI config soon and we can avoid merge conflicts later when Horizon releases if this change is also on master.

[2opremio]

I notice that this change was made only for the next Horizon release branch. Is there a reason this isn't on master? I may be making changes to the CircleCI config soon and we can avoid merge conflicts later when Horizon releases if this change is also on master.

Sure, I will create a PR to merge it back to master.