Only include sourceMessageID in ThreadInfo struct if it's set

Summary: Otherwise the thread inconsistency checks will think all of the ThreadStores are wrong and force every client to re-download the entire ThreadStore.

Test Plan: Use the React Native Debugger to make sure that sourceMessageID is missing for all non-sidebar ThreadInfos

Reviewers: palys-swm, KatPo

Reviewed By: KatPo

Subscribers: zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D612