broadcastPlaylistDumpsUpdated(playlist: Pick<Playlist, "id" | "isPublic" | "userId">,members: DatedPlaylistMember[]): void
Sends playlist_dumps_updated to the playlist's audience, each socket with
only the dumps its user may see, as GET would list them. The private dumps
in a public playlist stay hidden from everyone but their posters, and so
does their number: a client counts the dumps it is sent.
playlist: Pick<Playlist, "id" | "isPublic" | "userId">
members: DatedPlaylistMember[]