Calculate % of time blocked in credit flow

[michaelklishin]

We currently sample credit flow state for connections and channels. However, the status is not accurate: we report state as "flow" when it was in flow in the last 5 seconds. A much better strategy would be to report "flow utilisation", much like we do with consumer utilisation: a % of time when a process was blocked in flow.

[michaelklishin]

I'm not convinced this falls into the effort-low category, considering the ramp-up effort around credit flow that might be required.