Comprehensive node metrics traversal mechanism.

In order to get comprehensive metrics out of a flume node, we need a mechanism to deal with recursive traversals of information from a flume sink/source pipeline. The initial implementation was comprehensive but unnatural for json style interactions because it always flattened all metrics data into a single record. The implementation that was released in 0.9.0 was simple but lacked much of the detail required to understand the state of a node, was inconsistent with traversals and was not useful for debugging.

This implementation takes the middle road – it allows for rich individual reports for each connector and defers flatting and traversals to a separate step.