Platform API cleanup

[morozov]

Q	A
Type	improvement, deprecation
BC Break	no

The methods being reworked are defined in the AbstractPlatform sub-classes but do not belong to the Platform API:

1. OraclePlatform::assertValidIdentifier() is not used anywhere. The DBAL is not the source of truth about the validity of a given identifier.
2. OraclePlatform::getCreateAutoincrementSql() is only called from within the class and should be made protected.
3. OraclePlatform::getDropAutoincrementSql() is only called from within the schema manager. There's definitely some asymmetry with the above method. We can decide on how to improve this API later. For instance, get rid of the triggers in favor of identity columns.
4. The SQLServerPlatform methods in question are only called from within the class and should be made protected.
5. The SQLite user-defined functions don't need to be defined within the platform class.

[derrabus]

FIY: The ORM does not use any of those methods.