MultiDecoder improvements: Don't require nested sub-decoder config,
don't recycle packs unnecessarily, don't skip a decoder entirely if one
of a set fails to decode, don't hang if SandboxDecoder subdecoder
fails during initialization.
Got rid of unnecessary Decoders map, renamed Ordered slice to
Decoders.
Merge pull request #901 from mozilla-services/multidecoder-improvements

MultiDecoder improvements