services/horizon: Cap the minSeqAge precondition at MaxInt64

[Shaptic]

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 caps the minSeqAge precondition field (of type xdr.Duration = Uint64) value to be an int64.

This is an alternative to #4339.

Why

This ensures negative values don't make it into the database. We do the same thing with timebounds currently.

Known limitations

You can't use the values in the interval (MaxInt64, MaxUint64], but those are absurd anyway.

[sreuland]

I thought the approach taken on #4339 to preserve this data bit for bit as char blob was more compelling design, in principle, horizon stays neutral, clients get to interpret. Maybe avoid some edge cases, If clients chose to stuff a huge constant into minAge on a tx which has logical meaning in their application, and then they get something different from horizon, maybe that is worse?

[Shaptic]

Closing in lieu of #4339.