Implement C++ code to update thread unread status

Summary: Implement C++ code to update thread unread status in SQLite.

Test Plan: No tsting plan. Further diffs will use this functionality onn iOS and Android and provide appropriate testing plan.

Reviewers: palys-swm, atul, varun, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4235