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. Testing Done: Ran unit tests.