ir.sequence number_next does not work with postgresql 10.0

The new postgres release moved some sequence properties to a view: https://www.postgresql.org/docs/devel/static/release-10.html

Thus the code currently computing number_next uses incremented_by which is one of those moved fields.

I have a patch fixing the issue but it is not compatible with 9.6 so I propose to remove this field (which could be wrong anyway) and provide the functionality to set the sequence thanks to a wizard.