Improve transaction handling for CrateDB, which doesn't implement any

[amotl]

About

We are discussing what to do with the beginTransaction, commit, and rollBack methods of PDOConnection, whether and how they will emit warnings that CrateDB does not support transactions.

References

• Support for Doctrine DBAL 3 #122 (comment)

[amotl]

This patch configures TransactionIsolationLevel::READ_UNCOMMITTED as new default, where TransactionIsolationLevel::READ_COMMITTED had been the previous default.

• Transactions: Configure transaction isolation level and forward transaction methods to underlying PDO connection #156
• https://www.doctrine-project.org/projects/doctrine-dbal/en/4.3/reference/transactions.html