Something like the provenance of the lightsabre is, ultimately, irrelevant to those goals. At best, it's worldbuilding. It doesn't even rise to the status of a Macguffin, because in TFA at least, Luke is the Macguffin. He's the object that propels most of the action (but with which the audience doesn't really need or have a chance to engage). Luke is the Ark of the Covenant, or the microfilm, or the audio recording of the assassination attempt, or whatever. The lightsabre isnt' even that. It is, at most, a clue towards obtaining the Macguffin, like the headpiece to the Staff of Ra, or the photo of the well-dressed gentleman getting into the limousine with the suspicious box, or whathaveyou. It's a link in the chain that moves the characters closer to Luke (and also triggers the start of a separate journey for Rey, which presumably makes up a good portion of the next two films).
So, finding out where it's been, who has had it, and how it got from A to B is ultimately...irrelevant for the story of TFA.
That said, I wouldn't be surprised if SOME explanation is offered for its past in Ep. VIII, if only because it's already been highlighted that it's "a story for another time." That story, however, likely will not be all that well developed. A couple options:
1. It was retrieved by Luke, who gave it to Leia, who then gave it to Ben. When Ben killed a bunch of Luke's students, he left it as a "calling card" so that Luke would know for certain who had done this (although by then, as Kylo Ren, Ben would otherwise not be public about his identity). Luke then gives it away to Maz because it's too painful for him to keep.
2. Luke defeated Ben initially, when he started going dark, but spared Ben's life. Luke took the sabre from Ben (who himself had gone and dug it up), and then gave it to Maz.
3. Luke got the sabre and gave it to someone else after he rejected his destiny as a Jedi, following his failure as a teacher. That someone gave it to Maz (or gave it to someone who gave it to someone who.....gave it to Maz).
Alternatively, the issue may never be addressed, if the sabre's backstory doesn't end up relating to the central tale. There's no reason to go into detail if, for example, the sabre was recovered by Lando, who meant to give it to Luke, but then Luke went into hiding, so Lando gave it to Maz. Is that story relevant to the rest of the tale? No, not at all. So it doesn't need to be told in the film.
Basically, I wouldn't get my hopes up that the sabre backstory is going to be (A) particularly elaborate, or (B) explained in the film if the backstory is elaborate. It's probably worth maybe a 30-45sec explanation, and that's it.