PostgreSqlPlatform::getAdvancedForeignKeyOptionsSQL()

This reverts commit e3ab1276, reversing
changes made to f988b0a6.

- SERIAL and BIGSERIAL are not true types and can't be used in ALTER TABLE expressions
- When changing between int and bigint auto increment fields the default should no be dropped
  to preserve the link between the column and the sequence.

Serial fields already have a default value. By opting out of `notnull`
serial fields will just set the next value in the sequence.

 Closes #2436

fixes #2270