Same as much of the stack before, we need to add branching to support both LeagacyThreadInfos and MinimallyEncodedThreadInfos.
There's a lot of cut/paste here. I'm currently focused on getting to 0 flow issues across the codebase, but will come back and try to tidy these up before landing to avoid cluttering up codebase w/ noise.
[skip-ci]
Depends on D10278