(refactor) Bring the codebase up to date

The code has not been actively patched in ~2 months and both Iron and Rust have changed
quite a bit. This is my first round of refactors and everything appears to be green.
The major differences are the handling of Middleware in Iron. I changed the old
Middleware impl to one for BeforeMiddleware. The rest of the changes are centered
around making sure all tests can run, and are green.
Fix accidental replacement of text in Readme.md
Merge pull request #1 from launchdarkly/refactor/iron

Refactor/iron