API New $class arg for DataObjectModel->getModelTypeForField()

[chillu]

Avoids calling accessField() which in turn calls DataObject->obj(). It isn't directly interested in the actual return value, but rather the type of this return value. Due to the way the ORM and DataObject is built, we can't guarantee lazy evaluation of those return values. Some of them cause database queries. One example here is Versioned->versions(), which performs in-memory operations on a (lazy) DataList that turn it into a (concrete) ArrayList.

Performing database queries slows down schema generation, but more importantly the database might not be available when we build the schema as part of a deployment operation (rather than a dev or production environment). For example, when creating a deployment package on CircleCI which then gets transferred to the actual production environment.

Context: #388