Allow user to specify first block to download for the unchained index

[tjayrush]

This should be very simple -- just don't download for chunks prior to what the user specified.

The index would just be incomplete - - but that's what the user wants.

Trouble: if the user makes a manifest in such a situation, and then later wants to download earlier chunks, those old manifests would freshen prior to that.

Solution: store earlier encountered block in the monitor. If a chunk is earlier than that, pre-pend to the monitor the next time is scans.