Added reporting of message processing stats for each subdecoder and

in aggregate to the MultiDecoder.

Note that this does not capture the memory usage stats for any embedded sandbox decoders. It'd be tricky to do that efficiently with the current design. We've already been discussing redesigning our reporting infrastructure (introducing metrics objects that can encapsulate the reporting behavior) so I'm thinking we should land this improvement for 0.6 and do a bigger overhaul that better supports delegating reporting to the subdecoders after 0.6 is released.