This is a map of participants, indexed by
participant.id (a participant's peer
ID). This map emits an event whenever a participant present in the map emits an
event. For example, when a participant is added to this map, a
participantJoined event is emitted from the map. When a participant object
emits an event
videoUpdate, the map re-emits that event (provided the
participant is present in the map).