Check for transaction in Merge::execute()

Follow up of #3586760: Use composite key Upsert queries in core.

Problem/Motivation

Merge::execute() should be occurring in a transaction.

Proposed resolution

Option 1:

• Deprecate calling Merge::execute() outside of a transaction
• Do not call select() second time upon insert in case of insert failure AND in transaction

Option 2:

• Wrap Merge::execute() into a startTransaction/commitOrRelease structure, so that
• if a transaction is already active when Merge::execute() is called, the Transaction manager will manage as a SAVEPOINT/RELEASE SAVEPOINT operation
• if a transaction is not active when Merge::execute() is called, the Transaction manager will manage as a BEGIN TRANSACTION/COMMIT operation

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet