Should delivery filters be checked for noexcept-ness?

Delivery filters must not throw exceptions because they are called in `noexcept` context.

But delivery filteres aren't checked to be `noexcept` functors. Should this check be added to `so_add_delivery_filter`-family of methods (by using `static_assert`)?

It seems that this change may break compatibility with 5.8.0, so may be this change has to be postponed to 5.9?