[IMP] fields: update inverse fields lazily for performance

[IMP] fields: update inverse fields lazily for performance

When a relational field is assigned in an onchange, its inverse field is
updated in cache.  Reading the current value of the inverse field may be
costly, for instance in the case of a one2many field with thousands of records
as a value.  Instead, put in cache a SpecialValue that reads and updates the
field; it will be triggered only when it is accessed.