support/db: Remove ctx param from Begin/BeginTx/Rollback

[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 removes ctx param from Begin, BeginTx and Rollback methods of support/db.Session. It also updates Horizon to use the new method params.

Why

As described in the standard database/sql.DB.BeginTxx(ctx context.Context, opts *sql.TxOptions) method:

The provided context is used until the transaction is committed or rolled back. If the context is canceled, the sql package will roll back the transaction.

This behaviour interferes with defered Rollback calls because in such situation ROLLBACK query is sent twice (in the internal database/sql goroutine started in BeginTxx and in support/db.Session.Rollback. This can break the DB connection and make the DB to close the connection. This can be unexpected when the app doesn't want idle DB connections to be closed.

Known limitations

Other apps using support/db need to update the code. I couldn't find any other usages than the Horizon.