Throw exception on duplicate database names within a document

[alcaeus]

Q	A
Type	bug
BC Break	yes
Fixed issues	Fixes #924

Summary

As described in #924, this adds check to the metadata drivers to ensure that database names are not reused within a document. This check is done in the metadata drivers to ensure that it's still possible to re-declare a field from a parent class to change its mapping.

[alcaeus]

I was able to move the check to the metadata class, so we don't need to implement it in every driver 🎉

With this change, we're now throwing an exception if there are two properties with different names mapped to the same field in the database and both are not marked as notSaved. If they share the same property name we assume the original mapping is overwritten, while if either of them is marked as notSaved we also don't throw a mapping error since we won't have any conflicts due to the mapping. The sorted group relationships in the user document in our tests shows where this can be useful.

To allow a mapping references and embedded relationships as notSaved, I had to drop the NotSaved annotation and move it to the AbstractField annotation to allow references and embedded documents being marked as readonly. I'll need to come up with some more tests to ensure this works consistently, so I might extract that change to a separate pull request.

[jmikola]

Logic in mapField() looks sound. Left a few other comments.