Add more aggressive primary key matching

[jensyt]

The current implementation only matches primary keys if they are exactly equal, regardless of whether any secondary keys match. This might make sense, but it might also make sense to allow more aggressive top-level matching by looking at matches across all keys and comparing to some threshold.

E.g. two objects with keys [1, 2, 3, 4] and [2, 2, 3, 4] should possibly match, even though the primary key (1 vs 2) doesn't. If the threshold were set to match "at least 75%", this could be a match.