[lib] Clear MessageStore on freshMessageStore

Summary: We don't seem to use remove_all on the messages side, but we do on the threads side. Was this an oversight?

Test Plan: Haven't tested yet, mostly just asking posing this as a question

Reviewers: atul

Reviewed By: atul

Subscribers: tomek, abosh

Differential Revision: https://phab.comm.dev/D5659