Implement flat-file based message storage initialization
Summary: Implements dile and directory initialization for flat-file message storage.
Test Plan: Modify AppDelegate so that on app launch it prints the content of the directory TemporaryMessageStore was initialized with. Ensure that file ending with Unix timestamp is there.
Reviewers: palys-swm, varun, karol-bisztyga
Reviewed By: palys-swm
Subscribers: ashoat, Adrian, atul, yayabosh
Differential Revision: https://phab.comm.dev/D4392