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 <javier.martin.montull@cern.ch>
Co-authored-by: Jan Aage Lavik <jan.age.lavik@cern.ch>