Changeset r15232 refactored transactions so that all transaction state is...

Changeset r15232 refactored transactions so that all transaction state is maintained on the connection. This changeset continues that work, moving all transaction control to the connection, too. The transaction control functions in django.db.transaction are left as a generic way to easily apply a transaction control function based on a DB alias. Refs #9964.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15492 bcc190cf-cafb-0310-a4f2-bffc1f526a37