Table cartodbfy'ed without default seq value

[rafatower]

Steps to reproduce:

• Create an empty table:

CREATE TABLE bug_empty_table_no_seq (
    cartodb_id integer,
    the_geom geometry(Geometry,4326),
    the_geom_webmercator geometry(Geometry,3857),
    name text,
    description text
);

• Cartodbfy the table:

SELECT CDB_CartodbfyTable('bug_empty_table_no_seq');

• Try to insert an empty row:

INSERT INTO bug_empty_table_no_seq DEFAULT VALUES;

• Expected: the row is inserted without problem
• Actual:

ERROR:  null value in column "cartodb_id" violates not-null constraint
DETAIL:  Failing row contains (null, null, null, null, null).

The problem is that the table is created without the default sequence value. That breaks the editor and possibly other scenarios.

See CartoDB/cartodb#5392

[rafatower]

_CDB_Has_Usable_Primary_ID does not take into account if there's a valid sequence there. I think I got a fix for that particular case, but more profound changes might be required.