Merge upstream changes

Merge all upstream changes from: https://github.com/mozilla-services/heka-mozsvc-plugins/commits/dev

except 36fe7da278a8b626134c9f3e77743a835b50aa2a (https://github.com/mozilla-services/heka-mozsvc-plugins/pull/26) which would require using go-mock fork throughout clever private.

I think in a separate PR we can make sure to build the latest Heka and get all our changes into it, but this is the minimal change to get Heka private's build passing.