[lib] Include source message id in pending thread id

Test Plan:
Create a thread from pending thread and check if correct members were added.
Create sideber (on web and native) and check if it was correct.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: ashoat, KatPo, zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D811