Use READ COMMITTED by default for MySQL transactions

Problem/Motivation

By default, Drupal uses the REPEATABLE READ transaction isolation level - this results in lots of deadlock errors.

Drupal.org and many other high traffic sites use READ COMMITTED, so default to that instead. Drupal commerce has been recommended READ COMMITTED since before 2014.

Steps to reproduce

In some circumstances, drush cr + loading a single page in a browser can be enough to trigger a deadlock.

Proposed resolution

Do 2 things:

1. Add the possibility for siteowners to set the transaction isolation level in the database settings array in setttings.php;
2. Change the default transaction isolation level for new sites from REPEATABLE READ to READ COMMITTED.

Remaining tasks

TBD

User interface changes

In the database configuration for MySQL, MariaDB or equivalent databases in the installation proces can now the transaction isolation level be set.

API changes

The transaction isolation level can be set in the database settings array in setttings.php

Data model changes

None

Release notes snippet

TBD