Refactor: Improve set_count() behaviour.

[sumanthvrao]

This PR makes the set_count behaviour more consistent.

The commits are broken as:

• Rename a variable in set_count() function for better understanding
• Extracting and distinguishing (read vs unread) two functions for setting unread_counts in model. This commit also contains how we can now use the two functions in set_count() for setting stream, pm unreads.
• Improving the reading behaviours for muted messages (Where by we set there unread_counts as well). This commits also sets unread_counts of unread[topics] which we had not done previously.

[sumanthvrao]

@neiljp FYI. Rebased this ;)

[sumanthvrao]

Sure. Sounds reasonable :+1:

[zulipbot]

Heads up @sumanthvrao, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/master branch and resolve your pull request's merge conflicts accordingly.