das: Better metric for showing sampled range now that SamplingWindow is enabled

[renaynay]

Right now, sampled chain head refers to the tip of the sampled chain which, while accurate, does not reflect that the light node may not have sampled everything before the SamplingWindow.

There should be some sort of metric that reflects where the light node began sampling potentially - or some way to know whether a light node has actually triggered a sampling job over block at height N.

Needs to be fleshed out a bit more.

(from discussions with @jcstein)