CORE-4126 add sys msg reception tracking

The enumerated type user_acknowledgment_state was added. It represents
what a user has done with a given notification.

The system_notification_acknowlegments table has been modified.  It now
contains a state column of type user_acknowledgment_state.  The now
redundant deleted column has been removed.  The useless id column was
removed.  Finally, the primary key is now the pair of user and system
notification ids.