Those are needed to accommodate to API request needs without handling
sorting or pagination in Python.
Instead of adding four new arguments to get_objects interface, they are
consolidated using a single Pager object that is passed through the
_pager argument. The name uses underscore to avoid breaking objects that
want to pass 'pager' filter into SQLAlchemy. Hopefully, no objects will
ever have '_pager' attribute in their models.
Related-Bug: #1541928
Change-Id: I7dafc4dbd80f0ac35dbc2c2f30e56e441f5b1fc0