[lib] Don't assume that threads exist in thread store when merging new messages

Summary:
We don't store pending threads in thread store.

https://linear.app/comm/issue/ENG-9140/incorrect-handling-of-startreached-in-message-reducer

Test Plan: Check if creating thin and thick threads is possible.

Reviewers: kamil, inka

Reviewed By: inka

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13239