The extension manager tests were encountering a few errors caused by

changes to Django's database layer.
First, there were some errors coming from django-evolution using the

schema editor. The sqlite backend has been changed to error out if the

schema editor is invoked in the middle of an existing atomic

transaction, and django testcases are all run from within transactions.

To get around this, I've made it so the entire extension manager test

case disabled constraint checking.
Second, the allow_thread_sharing property was made read-only, and

changing it now requires using the inc_thread_sharing() and

dec_thread_sharing() methods. These have been added where appropriate.

Ran unit tests.