Introduce messages field in MessageStore during rehydration

Summary: This revision introduces empty messages field in MessageStore. This is essential change. The context why is here: https://linear.app/comm/issue/ENG-2133/sqlitecontextprovider-should-not-run-when-user-is-logged-out

Test Plan: Use messageInfoSelector in a place before SQLiteContextProvider dispatches action setMessageStoreMessages. Before this diff it will fail. I will not after.

Reviewers: tomek, atul, kamil, ashoat

Reviewed By: ashoat

Subscribers: ashoat, abosh

Differential Revision: https://phab.comm.dev/D5545