mixer: blend improvement
mixer: blend improvement

* Loads all models before blend/unblend to match correctly mixers
  with models.

* Commits results to DB only once per mixer to speed up load time.

Signed-off-by: Javier Martin Montull <ea6b80e26c65acc242d8916fb6eefa01b123cb69@cern.ch>
Co-authored-by: Jan Aage Lavik <8d55e006a1b29770f275a5c59a7341f17f8e69d5@cern.ch>

inveniosoftware/invenio,tiborsimko/invenio,tiborsimko/invenio,inveniosoftware/invenio