compositemixer: Solve deadlock on element release
compositemixer: Solve deadlock on element release

Change-Id: I700887fa3dbaf9622f17d5d6ee42b4c9cb542745

shelsonjava/kms-elements,Kurento/kms-elements,TribeMedia/kms-elements,shelsonjava/kms-elements,shelsonjava/kms-elements,TribeMedia/kms-elements,Kurento/kms-elements,Kurento/kms-elements,TribeMedia/kms-elements