Added support for savepoints to the MySQL DB backend.

MySQL provides the savepoint functionality starting with version 5.0.3
when using the MyISAM storage engine.

Thanks lamby for the report and patch.

Fixes #15507.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17341 bcc190cf-cafb-0310-a4f2-bffc1f526a37