Reimplement reporting infrastructure

[rafrombrc]

Right now every plugin can generate custom report data to be included in the dashboard, but there's no consistency or shared reporting infrastructure. This should be revisited to provide at least the following features:

• Message count, error count, and appropriately sampled processing duration values should be provided "for free" without the need for each plugin to implement this independently.
• MultiDecoders should be able to delegate to sub-decoders to extract reporting information from them while still being able to keep track of which data came from which sub-decoder.

[validname]

Have you thought of adding percentiles of plugin's processing times? Indeed, average processing time is not so useful as an answer on question "what time (e.g.) 85% of messages was processed?". And if you count average time on the infinite period, it becomes more useless...