Edited Feature: add model validation pass in odata model builder [437]

we should validate things like when users build edm models explicitly (not for the implicit model we build in query composition)
1) an entity type has key(s).
2) an entity type has a corresponding entity set.
3) all navigation properties are bound to an entity set.
4) a non-abstract entity type is bound to a non-abstract clr type.
5) all properties in an edm type have public setters and public getters in the backing clr type.