Provide API for listing which nodes in a workflow are active and how many requests they are handling

[mxgrey]

The current event tracing mechanism does nothing to report the current state of which nodes / requests are active. That information currently needs to be inferred by counting the message sent events. While that's technically viable, it may prove unreliable (sticky) if any events get lost or there's a late-joining situation.

We should provide a core crossflow API for querying which nodes are currently active and what requests they're currently processing.