The `PostgreSqlPlatform`, `SQLAnywherePlatform` and `SQLServerPlatform` classes have been renamed to `PostgreSQL94Platform`, `SQLAnywhere16Platform` and `SQLServer2012Platform` respectively.

When deprecating an old platform version, we can deprecate and later remove a specific class. Otherwise, there's no way to express the deprecation on the class level and the upgrade notes contain sentences like: "`XYZPlatform` and `XYZKeywords` now represent XYZ (version)".

also simplifies the code and now exactly matches postgresql search rules

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

Fixed inconsistent use of getSQLDeclaration method