DefaultEntityRepository cleanup

[maurei]

This PR fixes of several issues and a bunch of refactoring.

• fixes an issue with EF Core exceptions thrown when DefaultEntityRepository tries to attach entities to the DbContext that are already attached. This may happened when entities involved to a update or create request were already loaded in the dbcontext elsewhere (eg Resource Hooks). Closes Refactor AttachRelationships: prevent reattachment, support implicit remove #502. This fix is needed for the tests to pass in Hooks  #478
  • added consistent handling for tracked entities
• support for implicit removal of relationships. For this the IInverseRelationship service is introduced (actually it was cherry picked from the Resource Hooks PR). See Refactor AttachRelationships: prevent reattachment, support implicit remove #502.
• Refactored the AttachRelationships method and those associated to it such that
  • it doesn't also assign relationshipvalues in (only) the case of many-to-many
  • it no longer depends on IJsonApiContext for the values of updated relationships and attributes. Still uses it to identify which attributes/relationships to update. See DefaultEntityRepository: inconsistent data handling #519
• Marked UpdateAsync(TId id, TEnity entity) as obsolete, added a new one: UpdateAsync(TEnity updatedEntity) which actually uses the updated entity in the repository layer, as opposed to it just being passed along and being left untouched in the previous implementation. See DefaultEntityRepository: inconsistent data handling #519
• Refactored the fix that was included earlier to support complete replace for PATCH requests (see Update one-to-many through PATCH on resource not working #492 and Update many-to-many through PATCH on resource not working #494).
• Refactored the usage of the following properties of IJsonApiContex RelationshipsToUpdate, HasOnePointers, HasManyPointers. The first one has become an alias for the latter two, and the latter two are no longer populated with the actual relationship values as these are not used anymore in the repository layer. This is related to [draft] Feat/serializer context decoupling #512.
• No longer allowing PATCH requests that do not contain ID values in the request body. Fixes Bad PATCH request (without ID) does not return HTTP 400 #520