services/horizon - db clear command truncates all tables

[brianc-tbcasoft]

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

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.

Summary

Ingestion.ClearAll() truncates all tables instead of deleting by range.

Goal and scope

The goal is make db clear command operate as intended. Reference: #1371

Summary of changes

• added missing TableNames const declarations
• modified Ingestion.ClearAll() to truncate all tables
• added Session.TruncateTables() utility method
• modified TestClearAll() to test for all empty tables

Known limitations & issues

none

What shouldn't be reviewed

n/a

[bartekn]

LGTM! Can you sign the Contributor License Agreement?

[brianc-tbcasoft]

awesome. i've signed the agreement