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).