Extend TemporaryMessageStore interface with private auxiliary methods to manipulate paths

Summary: Adds and implements private methods to manipulate file names and create latest file to store messages

Test Plan: We can temporarily modify AppDelegate to invoke thos methods and check in the debugger that _getPath and getLockName return valid string and _updateCurrentStorage creates new file with expected name

Reviewers: palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4472