services/horizon: Fix ledger capacity usage in fee stats

[bartekn]

Summary

In protocol version 11 that implemented CAP-0005 the number of operations is used as the indicator of ledger capacity. /fee_stats endpoint is incorrectly using transaction count for ledger capacity stats.

Goal and scope

Rewrite a query that calculates ledger capacity usage to correctly report fee stats.

Summary of changes

The query that calculates ledger capacity in the last 5 ledgers has been rewritten. Because we don't have access to number of failed operations in history_ledgers table (#1385) we first run subquery that returns number of operations for ledgers in question along with max_tx_set_size for each ledger and then calculate the capacity in the parent query.

Known limitations & issues

• The query can be simplified when add failed operation count to ledger resource #1385 is implemented.

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.

[ire-and-curses]

LGTM