Deprecate Key2StringMapper

ISPN-3471 is still valid: Key2StringMapper is too low-level, as it has to deal with user objects or WrappedByteArray depending on the memory configuration.

For the vast majority of cases, a key mapper implementation that uses the same marshalling for the keys as for the values should be good enough. It may also use base64 encoding for databases that don't have a `VARBINARY` equivalent, but I'm not sure there are any.

The only use case I see for a custom Key2StringMapper and a VARCHAR key column is for external tools to look at the database directly, but we never supported that.