Provide means to open a transaction through TransactionManager

TransactionManager currently doesn't support opening a transaction, just queuing changes to be performed. This isn't sufficient, as we need a way to open a transaction to ensure it's snapshot remains the same throughout the transaction. This commit changes TransactionManager to provide an explicit interface for managing a transaction similar to other databases. Begin opens a transaction and the returned transaction can be operated upon to stage changes to be ultimately committed or rolled back. This commit updates the interface only but doesn't yet do any synchronization otherwise.

Part of #4792 (closed)