Use Apache commons-lang3 FieldUtils instead of ad hoc reflection

There are a number of places in the Nuxeo code where we need to use reflection to read/write a field otherwise inaccessible (private or final).

Instead of using ad-hoc reflection, we should use FieldUtils which makes the code shorter.

While doing this cleanup, remove the cases where reflection is not actually needed (or change Nuxeo code to make the field usable directly).