support/db: Support concurrent queries in a transaction

[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 Synchronized flag to support/db.Session. When set to true and Session runs a transaction all Exec* and Select* methods are protected by mutex what allows running them in multiple goroutines.

This is an experimental feature (see below) and not a breaking change (default is false).

Why

Postgres protocol does not allow sending Exec query results if previously sent Query haven't been fully read. This issue manifested itself when a PR that's sending read and write queries in multiple goroutines was merged.

More info: lib/pq#81 lib/pq#635

Known limitations

• It's possible that it will not be needed if we decide to remove concurrency from ingestion pipeline (see Add Synchonized flag to db.Session #1983). We are adding this to unblock development of new ingestion processors with a more readable code (currently all database processors are merged into one big processor that is hard to read and test). Splitting DatabaseProcessor will be done in a separate PR/PRs.
• During Horizon Team meeting we agreed to create a new SynchronizedSession struct embedding db.Session inside that would not be placed in a shared support package. The problem is that history.Q embeds db.Session inside. Changing this to db.SessionInterface requires updating a lot of files that create history.Q objects, tests and mocks. Given that we may want to revert this change in the future, the change in this commit seems to be simpler.

[abuiles]

👍