Add better support for expressing saturation in statemaps

[prakashsurya]

It would be great to expose better support for expressing saturation in a statemap.

For example, the vdev-statemap.d script allows one to determine when a given vdev is "active" (reading, writing, or both), but it doesn't allow one to determine "how active" the vdev is; i.e is it servicing a single write, or multiple writes? if multiple, how many?

There's an example of this from here.

While I haven't seen the data used to generate the visualization (@bcantrill, can you link to the script used to generate that data?), I presume each color is a distinct state in the statemap. This can work, but as alluded to here (assuming I correctly understood the reply), it might be better to allow the state to be a scalar value, expressing some sort of magnitude of that given state; e.g. the state can be "write", and have a scalar to represent the number of outstanding writes at that given time.