[lib] Use RawThreadInfos type everywhere

Summary:
In some places we were using RawThreadInfos, in others we were using { +[id: string]: RawThreadInfo }, and in others we were using { +[threadID: string]: RawThreadInfo }.

This is just for convenience when refactoring. Searching for RawThreadInfos or using "go to usages" will actually show us all uses of this type: https://blob.sh/68308a.png

Test Plan: NA

Reviewers: ashoat, ginsu, tomek, rohan

Reviewed By: ashoat

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9787