Traversal can end up keeping large amounts of path metadata

[willscott]

as seen in the legs synchronization https://github.com/filecoin-project/go-legs/pull/126 we ended up seeing memory exhaustion at traversals of ~10,000 depth, and ended up segmenting the traversal process into smaller selectors limited to 2000 levels of recursion as a stop-gap.

That seems really small and like something has gone wrong at the traversal level to warrant that.