Optimize DB queries using "fields"

[smaugho]

When the queries are executed in the Local DB, all the fields are always loaded (at least that a row query is done). To avoid having to write a Raw query, it would be good to use the "fields" passed to the @Model annotation or loader in order to execute the operation only over those fields.

When "$PutModel" is executed, it should use also the fields, and if they exists, it should make an Update() of those fields and not all the class.