Remove DecoderSet abstraction (and `DecoderSet()` method in the
PluginHelper interface) and replace it with Decoder and DecoderRunner
methods that will create and return a decoder or a runner by name.
MultiDecoder needs to support WantsDecoderRunnerShutdown for the
subdecoders.
Simplifies the code to always use the DecoderRunner channel for
decoding.
Dealing with unwrapped Decoders is tricky, so let's give a warning, and
let's not expose the Decoder method on PluginHelper.
Merge pull request #506 from mozilla-services/no-mo-decoderset

Remove DecoderSet abstraction (and `DecoderSet()` method in the