This is a meta ticket to track progress on improving transaction handling:

• Transactions should automatically rollback on any Throwable (refs Transactions not rolled back when fatal errors are thrown in PHP7 #8997)
• We should have an easy way to change the transaction isolation level (refs 3.0 RFC: Transaction Isolation Level Abstraction #3136)
• Add a onConflict to the Query API. This method will abstract the specifics of each driver for handling constraint problems when updating or inserting (refs Duplicate entry on INSERT for concurrent database sessions #7458). This will open the door for having a upsert method in tables.