services/horizon/internal/actions: Wrap SSE handlers in repeatable read tx

[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.
  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

• Execute SSE handlers in repeatable read transactions
• Obtain account info from stellar core db using repeatable read transaction

Why

Close #2331

I noticed Some extra balances error in /accounts/{id} stream. At first, I thought that it was the inconsistency issue propagated from stellar-core (we build Account object using a few DB queries without REPEATABLE READ transaction). But then I realized that we don't use REPEATABLE READ transaction in Horizon DB while streaming.

We should update the streaming code to start a REPEATABLE READ transaction (if not in a transaction already) before calling an action function and then rollback (only if not in a previously started transaction).

...

We've seen another instance today for non-streaming request and the reason is getting data from core in multiple requests. So we should probably add start a new REPEATABLE READ transaction when getting data from Stellar-Core.

Known limitations

[N/A]

[abuiles]

👍

[abuiles]

@tamirms under which scenarios would session be nil?

[tamirms]

all horizon handlers should have a non nil session. This check is only included because some of the handlers in services/horizon/internal/stream_handler_test.go do not include a session in the request context