SegmentFetcher: use new diversity-seeking share-selection algorithm, and

SegmentFetcher: use new diversity-seeking share-selection algorithm, and
deliver all shares at once instead of feeding them out one-at-a-time.

Also fix distribution of real-number-of-segments information: now all
CommonShares (not just the ones used for the first segment) get a
correctly-sized hashtree. Previously, the late ones might not, which would
make them crash and get dropped (causing the download to fail if the initial
set were insufficient, perhaps because one of their servers went away).

Update tests, add some TODO notes, improve variable names and comments.
Improve logging: add logparents, set more appropriate levels.