[unstructured] Minor refactorings

[bettiolm]

• Remove adjacency member from unstructured user concepts (domain descriptor);
• provide concept for receive_domain_id_generator functor. So far it is implemented directly in the test / benchmark application code. By definition the implementation is application-dependent, but still this could help users in writing their own. Also just adding it to the documentation might be an option;
• re-use some structs from communication_object_2 (field_info and buffer) instead of defining new ones; they can be reused at least for the send part;
• offset of serialized buffer should not be needed anymore. This has to be verified first, and, if true, offset should be removed both from field_info and from field_info_ipr, in which actually it does not even refer to a serialized buffer.