Implement write operation for flat-file based message storage

Summary: Implements write operation for flat-file based message storage. It aither appends message to main storage file, or creates temporary file with random name if semaphore acquisition failed.

Test Plan: Use implemented method in NotificationService to write to main storage.

Reviewers: palys-swm, varun, karol-bisztyga

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4393