[lib] refactor Message Store Operations to use generic type

Summary:
Make logic part of the general ops interface.

Note: Messages has a lot of different converters of a different types, so this could not be fitted in generic spec, so I chose the most general converter.

Depends on D8621

Test Plan: Flow

Reviewers: michal, tomek, atul

Reviewed By: atul

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8622