Backfill fee stats data to make the client compatible with horizon 1.X and 0.X.

[abuiles]

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

Make the client compatible with fee stats data from Horizon 1.X or 0.X.

Why

There are a couple of breaking changes in the Horizon response which we are handling in horizonclient 2.0 in a backward compatibility manner, for example, being able to read a number or a string in some of the response fields. It allows a smooth transition between the new and old versions of Horizon.

This PR follows the same approach for fee stats, Horizon is dropping all the _accepted_fee fields in fee_stats but instead of adding this breaking change in the next release of the client, we'll make it backward compatible, so people can still use p99_accepted_fields in their code and have time to upgrade up to horizonclient 3.0.

Known limitations

[TODO or N/A]

[ire-and-curses]

Just a question about the tests - probably I'm misunderstanding something

[ire-and-curses]

Shouldn't these fields be kept, not removed? These are the ones being backfilled right, so we'd expect them in the response, wouldn't we?

[abuiles]

@ire-and-curses no, they are being removed from the response and then we backfill from the values in max_fee -- but the backfill doesn't happen in the response but inside the FeeStat struct which is what we do here
https://github.com/stellar/go/pull/2290/files#diff-a12e7e90b7de9b52aa97a8dca1ea276cR324