Fix unique indexes in CREATE TABLE statements on SQL Anywhere

SQL Anywhere seems to distinguish between unique indexes and unique constraints. Unique constraints do not allow `NULL` values while unique indexes do.

*A UNIQUE constraint is not the same as a unique index. Columns of a unique index are allowed to be NULL, while columns in a UNIQUE constraint are not. Also, a foreign key can reference either a primary key or a UNIQUE constraint, but cannot reference a unique index since a unique index can include multiple instances of NULL.*

The current implementation create unique constraints instead of unique indexes during `CREATE TABLE` statements which causes ORM to fail as all nullable columns in a unique constraint silently get converted to `NOT NULL`.
This PR replace unique constraints by unique indexes in `CREATE TABLE`.