next round of refactors and fixes.

 - moved default context values inside OpenBazaarContext.get_defaults()
 - new OpenBazaarContext.create_default_instance(), used now on test_transport.py -> get_mock_open_bazaar_context()
 - imports cleanup
 - '--database' is no more, now it's self explanatory and unambiguous '--db-path'
 - When shutting down we now have a mutex to make sure two SIGTERM and SIGKILL don't call the application.shutdown() routine at the same time.
 - style fixes.