* service: start room composite egress configured in a room preset
StandardRoomAllocator.CreateRoom merged a room preset into a copy of the
request and discarded it. RoomInternal has no field for room composite
egress, and getOrCreateRoom started it from the caller's original request,
so a room composite egress from a preset was never started.
RoomAllocator.CreateRoom now also returns the request with the preset
applied, and getOrCreateRoom starts the egress from it.
* service: return ErrEgressNotConnected instead of calling a nil egress launcher
getOrCreateRoom started a room composite egress without checking that an
egress launcher exists. RoomService.CreateRoom rejects egress on the request
when there is none, but not egress from a room preset or a token's roomConfig.
Martin Njalale