[lib] Replace LegacyThreadInfo with ThreadInfo in lib/shared

Summary:
Updated message-spec and then made necessary changes.

We're now at 94 instances of LegacyThreadInfo down from 700+...

Depends on D10136

Test Plan: CI/flow/etc.

Reviewers: ashoat, ginsu, tomek, rohan

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D10137