Moved by her own deep hunger to belong — to have an identity that matters and a tribe of her own — Alex Wagner, author and host of Showtime’s “The Circus,” set off on a quest to find the truth about her family history. It became the memoir Futureface, and it took her from Burma to Luxembourg, from ruined colonial capitals to high-tech genetic labs. But as she closed in on the mystery of her own ancestry, she began to grapple with a deeper question: Does it even matter?