IDENTITY/GENERATED v31

Hi,

here comes the next version. Changes:
* Implemented ALTER TABLE ... DROP IDENTITY
and SET GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY
* Extended documentation and testcase for the above.
* pg_dump now dumps the proper ALTER for such columns.
Reintroduced the pg_dump feature to dump
OVERRIDING SYSTEM VALUE for INSERT and
COPY statements if the table schema contains any
GENERATED ALWAYS column.
* There can be only one IDENTITY column in a table at any time.
This is checked on CREATE, ALTER TABLE ADD and
ALTER TABLE SET ... IDENTITY and required for SQL:2003
compliance.
* On update, the GENERATED columns' late generation
of DEFAULT value is also implemented. The GENERATED
ALWAYS AS ( expr ) still only deals within the same
limits as regular DEFAULT.
* Implemented a check so CHECK constraints cannot
reference IDENTITY/GENERATED columns.
It is checked on CREATE AND ALTER TABLE
ADD CONSTRAINT and ALTER TABLE ADD COLUMN.
It makes sense because IDENTITY and GENERATED
columns has to be assigned after all other columns were
assigned with values and validated via constraints.

I hereby declare the IDENTITY part feature complete.
Please review.

Best regards,
Zoltán Böszörményi