Improving code readability on lib/_http_agent.js

As per proposal #60, I have begun cleaning the code to create a more readable codebase.

I feel, and hope others agree, this will allow a more diverse and wider community to begin contributing and collaborating with io.js, reducing the learning curve and amount of developer documentation required for the codebase, equally making it easier to test and understand what is going on under the hook.

I will continue to clean the codebase into a self documenting one where possible, and encourage others to do the same. 
Could this be accomplished with a comment rather than creating another ``var`` to keep track of? I find the ``don't leak`` to be more sloppy than this particular change.

I don't like this pattern... but I applaud the effort of improving readability and variable naming as you have above. :+1: