Add unique constraint option to hasMany fields

[pborissow]

Consider the following model:

    UserGroup: {
        fields: [
            {name: 'name',          type: 'string', unique: true, required: true},
            {name: 'description',   type: 'string'}
        ],
        hasMany: [
            {model: 'User',         name: 'users'}
        ]
    }

This will create a table like this:

CREATE TABLE USER_GROUP_USER (
    USER_GROUP_ID BIGINT NOT NULL,
    USER_ID BIGINT NOT NULL,
    CONSTRAINT FK_USER_GROUP_USER FOREIGN KEY (USER_GROUP_ID) REFERENCES USER_GROUP(ID)
        ON DELETE CASCADE ON UPDATE NO ACTION
);

There is no unique constraint for users in the user group table.

We should add support for a new "unique" keyword option to hasMany fields like this:

       ...
        hasMany: [
            {model: 'User',   name: 'users',   unique: true}
        ]
    ...

With the "unique" keyword we should create a unique index like this:

CREATE unique INDEX IDX_USER_GROUP_USER ON USER_GROUP_USER(USER_GROUP_ID, USER_ID);