more precise progress indication for MDMF retrieve

[davidsarah]

From the commit message for 9756146d61673552:

More improvements to consider:

• progress is currently 0% or 100%: should calculate how many segments are involved (remembering retrieve can be less than the whole file) and set it to a fraction
• "fetch" time is fuzzy: what we want is to know how much of the delay is not our own fault, but since we do decode/decrypt work while waiting for more shares, it's not straightforward