Add ability to validate shipping addresses more stringently (via 3rd party api, for example).

Currently this feature is inactive, and tapping into it requires to replace the constraint validator with a proper implementation. This could be made easier via config..

You may also notice that I am doing some manual work to only validate strongly when the superficial validation passes. This can supposedly be done via the "group sequence" validation feature (http://symfony.com/doc/current/book/validation.html#group-sequence) but it actually does not work properly.