Bug 978876 - Part 0: add set utilities to CommonUtils. r=gps
Bug 978876 - Part 1: handle declined engines in desktop Sync meta/global. r=gps
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Bug 978876 - Part 2: refactor out meta/global upload.
Bug 978876 - Part 2: refactor out datatype choice. r=markh
Bug 978876 - Part 3: mark engines as declined after making a datatype election. r=markh