CMFActivity.Activity.SQLDict: Drop single-use variable.
CMFActivity: Drop no-op registerActivityBuffer API.
CMFActivity.ActivityBuffer: Drop unused property.
CMFActivity.ActivityBuffer: Switch to defaultdict.
CMFActivity.ActivityBuffer: Drop unused import.
CMFActivity.Activity: Do not check message presence when registered.

This should never match, because published API does not provide access to
Message when registering it (this is intentional).
Also, this search is O(n), which makes activity creation complexity
O(n**2).
CMFActivity.ActivityBuffer: Do not define unused/duplicate properties.

_registered is already declared on TM superclass.
_p_* are only useful on subclasses of persistence.Persistent. TM is not.
CMFActivity.ActivityBuffer: Drop duplicate activity reference.

Activity is expected to be appended to message_list_dict's value already,
no need to append it to queued_activity.
In turn, this allows getting rid of _prepare_args.