Reorganize local storing of Tx

[plyhun]

Abstract

One should never store a thing in ThreadLocal, since we operate async contexts, not threads.

Checklist

General

• [x] Added abstract that describes the change
• [x] Added changelog entry to /CHANGELOG.adoc
• [x] Ensured that the change is covered by tests
• [x] Ensured that the change is documented in the docs

On API Changes

• [x] Checked if the changes are breaking or not
• [x] Added GraphQL API if applicable
• [x] Added Elasticsearch mapping if applicable

[plyhun]

Postponed by now.