Correctly handle owned sequences with extensions

Correctly handle owned sequences with extensions

With the refactoring of pg_dump to handle components, getOwnedSeqs needs
to be a bit more intelligent regarding which components to dump when.
Specifically, we can't simply use the owning table's components as the
set of components to dump as the table might only be including certain
components while all components of the sequence should be dumped, for
example, when the table is a member of an extension while the sequence
is not.

Handle this by combining the set of components to be dumped for the
sequence explicitly and those to be dumped for the table when setting
the components to be dumped for the sequence.

Also add a number of regression tests around this to, hopefully, catch
any future changes which break the expected behavior.

Discovered by: Philippe BEAUDOIN
Reviewed by: Michael Paquier