Postgres SERIAL is not a post-insert identifier generation strategy

That's when using a sequence explicitly. `SERIAL` is using a sequence implicitly. I'm looking at [`Doctrine\ORM\Id\BigIntegerIdentityGenerator`](https://github.com/doctrine/doctrine2/blob/9280a4621df04bcd625da2a111bb12bd5fda9260/lib/Doctrine/ORM/Id/BigIntegerIdentityGenerator.php#L50-L64).

When using `SERIAL` fields, then this generator is being used, whereas using a sequence explicitly causes the [`Doctrine\ORM\Id\SequenceGenerator`](https://github.com/doctrine/doctrine2/blob/9280a4621df04bcd625da2a111bb12bd5fda9260/lib/Doctrine/ORM/Id/SequenceGenerator.php) to be used.