exp/ingest/ledgerbackend: Ensure tx meta is v2 when getting protocol version <10 ledgers

[bartekn]

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

This commit adds a code that checks if tx meta version is v2 for protocol version <10 ledgers.

Close #2095.

Why

As explained by @MonsieurNicolas in #1902:

There is some missing meta in protocol version older than version 10. This impacts transactions with one time signers (that get removed after operations in v9 and earlier; and during signature verification in v10 and later). Those changes are invisible in the meta which may confuse certain ingestion processors that keep track of signers on accounts.

It's possible that users will use stellar-core without SUPPORTED_META_VERSION=2 and in such case ingestion system can process state changes incorrectly.

Known limitations

I realized we don't have tests in exp/ingest/ledgerbackend. Would be great to add them in a separate PR.

[abuiles]

👍