* Added support for exclusive locking for store keys with context managers and asyncio coroutines.
* Added several unit tests for store exclusive key locking.
* Refactored librarian to obtain an exclusive lock on LIBRARY_ACTIVE key.
* Refactored engine to merge the execute_join_gateway task into the common queue (yay!), using exclusive key locking for a session's parent record.
* Added stub Django project space for pending UI.