webchat example: use file on disk rather than in-memory database. not sure why, but :memory: now gives me OperationalError: no such table django_session

pool: rename GreenletSet to Group. The old name is available as an alias (will be removed in the future)

also use a message that is a bit different from standard in this case

core: make sure we never incref events more than once and never decref them more than we increfed them. Thanks to Cyril Bay.

This fixes Cyril's test as well as test__server_pywsgi.py/test__server.TestDefaultSpawn.test_error_in_spawn which DECREFed event twice (detected by python-dbg).