Merge from upstream

Merging from upstream.

Reviewed by Steve Salevan - (via email)

Please review changes of upstream twemproxy merge to tumblr/twemproxy

[1] Diff of changes between tumblr/twemproxy/master and twitter/twemproxy/master
      - These are the changes that we introduced.

[2] Diff of changes between tumblr/twemproxy/master and tumblr/twemproxy/merge-from-upstream
      - Changes introduces from upstream (new version of twemproxy)

[3] Diff of changes between twitter/twemproxy/master and tumblr/twemproxy/merge-from-upstream
     - This diff should be the same as [1], proof that we are keeping our patches intact.

There was a mention of patches that did not apply, and I did not find that to be the case. I will wait until I hear a definite we are happy with the changes before I merge tumblr/twemproxy/merge-from-upstream into tumblr/twemproxy/master (diff [3])

[1] https://github.com/tumblr/twemproxy/compare/twitter:master...master
[2] https://github.com/tumblr/twemproxy/compare/master...merge-from-upstream
[3] https://github.com/tumblr/twemproxy/compare/twitter:master...merge-from-upstream