Discriminator should be read-only

[AndriySvyryd]

No description provided.

[ajcvickers]

Make discriminator read-only by default, but allow explicit configuration back to read-write.

[lajones]

Note: after discussion with @ajcvickers the comment above does not mean "add an API on DiscriminatorBuilder which would call down and make the property read-write"; it means "rely on existing APIs to do that, whether that's Metadata API or some sugar we add later to allow any property to be set read-write if it is currently read-only, just don't implement this issue in such a way that it makes the discriminator's property never be settable to read-write". (Extra note: looks like we already have that sugar - see IConventionPropertyBuilder.BeforeSave and .AfterSave).