Handle `read_message` events from server.

On most days, I have both web-app and ZT logged into chat.zulip.org simultaneous, open in different windows. Its often that I read messages in ZT and see the messages and unread_counts updated correctly in the web-app. This indicates that read_message events from ZT to server are working as expected.

However, When I read a message in Web-app, The unread_counts and the messages remains unread(un-decreased) in ZT. This indicates we aren't handling read_message events from server to ZT.

We should potentially handle this consistently across different clients <----> ZT. (bi-directionally)