ingest: reduce memory consumption of CheckpointChangeReader

[tamirms]

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.

What

Reduce memory consumption by about 1GB when iterating through ledger entries during state verification and state rebuilds. This reduction in memory is achieved by:

1. Removing claimable balance and offer ledger keys from the visited set when encountering the init entry.
2. Using map[string]struct{} to store ledger keys instead of map[string]bool{} (see https://stackoverflow.com/questions/57620170/can-you-have-a-map-with-no-value-in-go )

The first optimization takes advantage of the fact that claimable balance and offer ids are globally unique. Once a claimable balance or offer is deleted it is impossible to restore it or create a new claimable balance / offer with the same id.

Known limitations

[N/A]

[sreuland]

lgtm, left comment for consideration in adding test coverage if possible