Issue #146: When using inheritance, you can specify, which ID to use

When storing objects with inheritance, it should be possible to choose, which ID (parent or child class) should be used for new object. For example:

public EntityA() {
    @Id
    long id;
    String field1;
}

public EntityB() extends EntityA() {
    @Id
    long id;
    String field2;
}

In this case and current version of ODB, ID will be generated from EntityA. It should be possible to trigger generating ID from child entity, for example like this:

public abstract EntityA() {
    String field1;

    public abstract long getID();
    public abstract void setID(long id); 
}

public EntityB() extends EntityA() {
    @Id
    long id;
    String field2;

    public long getID();
    public void setID();
}

In this case new objects will get their ID from child class and can use individual sequence generator settings.