Allow `transaction-isolation` level to be configured

[ElijahLynn]

https://www.drupal.org/project/drupal/issues/2833539#comment-12500749

Would like to discuss setting transaction-isolation to READ-COMMITTED by default as it solves a random, inconsistent flicker "SQLSTATE[40001]: Serialization failure: 1213 Deadlock" in some cases.

The default is currently:

mysql> show variables where Variable_name = 'tx_isolation';
+---------------+-----------------+
| Variable_name | Value |
+---------------+-----------------+
| tx_isolation | REPEATABLE-READ |
+---------------+-----------------+

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[geerlingguy]

This should at least be configurable.

See Drupal's docs on it: Setting the MySQL transaction isolation level