dumpFiguresFor(members: DatedPlaylistMember[],playlist: Pick<Playlist, "userId">,viewerId: string | null | undefined):
Returns what playlist says of its dumps to the viewer, members being
all of them: the dumpCount and lastDumpAt that
VISIBLE_DUMP_FIGURES reads in a query. The WebSocket service puts
them on the playlist each socket is sent.
members: DatedPlaylistMember[]
playlist: Pick<Playlist, "userId">