conference: fix crashes on leave

This patch fix several problems:
+ participants_ must be protected and not iterated while accessed
+ getVideoMixer() do not generate a video mixer, this is only
managed at one place.
+ Remove setConfId, enter/exitConf are here for this and this
is causing the videoMixer to not be destroyed in time, because
exitConference() was called after setConfId(""). This was causing
crashes because the videoMixer will update for the last participant
with the error: "The call is not bound to any conference"

GitLab: #660
Change-Id: Ic60bc7377b0315f7e2906ab03a7653381436180c