Adding comments to clarify cleaner policy in DarkCanaryChannelHandler.

The cleaner policy ensures that that Request objects are dropped from
the ClientChannelHandler.requestMap before they are dropped from
the DarkCanaryChannelHandler.requestMap, ensuring that dark responses
are never propagated upstream. This commit clarifies how this works.
Bump version
Merge pull request #52 from linkedin/dark-canary-fixes

Dark canary code patches