services/horizon/internal/txsub: Fix false positive bad sequence error when submitting transactions

[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.
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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,
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Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
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  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

Close #2628

Horizon can respond with a false positive bad sequence error when Horizon's ingestion lags behind Stellar Core. To avoid sending back this misleading error we should check that Horizon's ingestion is consistent with Stellar Core before checking whether the transaction exists in the Horizon DB.

We are able to check that Horizon's DB is consistent with Stellar Core indirectly by observing the sequence number of the source account of the transaction. If the sequence number of the source account is below the sequence number set in the transaction, then we know Horizon is still behind Stellar Core. Once the sequence number equals or exceeds the transaction sequence number we know that we are consistent with Stellar Core at the point where Stellar Core returned the bad sequence error.

[bartekn]

LGTM! We should include it in 1.7.0 - I updated the base branch.

[bartekn]

Please update CHANGELOG.