[CommCoreModule] implement C++ MessageStore

Summary:
Grouping logic in one class to make it cleaner and reuse some code.

Depends on D8659

Test Plan: Build iOS and Android and play with messages to make sure functionality still works.

Reviewers: michal, jon, atul, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8662