Merge branch 'release-v1.4.0' of github.com:matrix-org/synapse into develop

changelog.d/6141.bugfix

changelog.d/6145.bugfix

changelog.d/6146.bugfix

synapse/storage/events.py

synapse/storage/events_bg_updates.py

synapse/storage/events_worker.py

...ma/delta/56/destinations_retry_interval_type.sql.postgres

synapse/storage/schema/delta/56/redaction_censor2.sql

synapse/util/retryutils.py

tests/storage/test_redaction.py

tests/storage/test_transactions.py