Edits to the SystemIntegration chapter

[patricknaughton01]

Some edits to the SystemIntegration chapter:

• Combined code snippets in the multithreading example code to make it more concrete. I think the original was a bit difficult to follow due to a lack of specificity.
• Added a note about multiprocessing and the GIL to add context to the use of multithreading in Python.
• Added a link to the socketserver library where relevant.
• Minor terminology changes.