das: `DASer` should not iterate (fetch) headers of heights that can be skipped

[renaynay]

Currently, DASer iterates over (loads) all headers (even those that are outdated) when samples the chain. The DASer should be able to know the height of the "tail" height (oldest header in the chain that needs to be sampled currently) and request that instead of having to request every single header and check its timestamp to see if it's outdated as that's inefficient.

This is tech debt left remaining from the initial implementation of samplingWindow inside of the DASer.