Investigate (and document) transaction isolation

[Nemo64]

dbal defines Doctrine\DBAL\Connection::setTransactionIsolation which allows to set the transaction isolation to 4 different levels. https://www.doctrine-project.org/projects/doctrine-dbal/en/2.10/reference/transactions.html

This function runs a SET SESSION TRANSACTION ISOLATION LEVEL query in the background which should fail over the rds-data api (I haven't tested yet).

It doesn't look like it is possible to set the isoluation level within a transaction which is the only way to get something resembling a state. The documentation says that you can't change the transaction characteristicts during a transaction.

So I should probably document that it isn't possible to use that feature and maybe adjust the platform definition so that it isn't even tried.