Move read checks for attributes and relationships to IncludeMerger

[andrus]

IConstraintsHandler is a leftover from the old days. We can do some of its security checks on the spot inside IncludeMerger when we are assembling ResourceEntity. Let's make this refactoring that would avoid inclusion of attributes and relationships that will later need to be excluded.

[andrus]

closing for now. inheritance-based permission checks may require an entirely different solution