Make presence servermore maintainable by using standard ExpressJS naming

My first interesting changes to this server were very hard to debug until I eventually realized that what our code refers to as a server is completely different from how the word server is generally used in both ExpressJS docs and samples both in the official repo and in the wild.  They refer to what we're calling a server as an "app", and use the word "server" to refer to something different.

This made understanding ExpressJS docs and reusing code and examples seen elsewhere very difficult.

In order to make the code more approachable, maintainable, and easier to include snippets from the web, this PR offers up a renaming to the more typical conventions.  This probably wants @tOkeshu to review it, as he has more context with this coding and the naming choices than other folks.