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