Raise error for jdbc schema parsing with ambiguous tables

Using an ambiguous unqualified table causes problems such as models
having a composite primary key with the same column name multiple
times.  In general, it is a bug and Sequel should raise an Error
so users know to qualify the table.
to_json :naked option always true

There is no reason to include the JSON.create_id field in the to_json
output, since Sequel no longer creates objects automatically from
json.  This code is also the only code that depends on specific
features of the ruby json library.
Remove JSON.create_id handling from from_json

For similar reasons as the previous commit, the JSON.create_id
should no longer be used, so it should no longer be handled
on input.  If users must handle it, they can add a setter that
ignores the value.