scanner: Fail if 'since' is higher than the interface version

If an event or request have a "since" attribute that is larger than
the version of the interface it is in, fail with an explaining error
message.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>