Make value converting nulls internal for 6.0

[ajcvickers]

In 6.0 we implemented Allow HasConversion/ValueConverters to convert nulls. However, this has proved to be very problematic in practice with many pitfalls. For example:

• Value conversion to null in the store generates bad queries
• Value conversion from null in the store generates bad queries
• Value converters do not handle cases where the database column has multiple different values that convert to the same value
• Allow value converters to change nullability of columns

These are not trivial issues and for the query issues they are not easy to detect. This makes the feature somewhat a pit-of-failure.

Therefore, we are proposing to mark these APIs as internal for 6.0. This still allows people to use them, but behind an analyzer and documentation warning. (This is similar to marking them as experimental--see EntityFrameworkExperimentalAttribute) We can then make them fully public in a future release after gathering more feedback and possibly making improvements.

[ajcvickers]

@Pilchie @JeremyLikness FYI. We can discuss as needed.