- Fix (again) partial piece avoidance for seeds
   - Fix several partial piece (temp file) leaks,
     some uncovered by previous rarest-first fixes, some in end game
   - Don't lose all DHT peers if we stop quickly (backport from update branch)
   - Explore a kbucket if it's less than 3/4 full (backport from update branch)

  which is a rewrite of the netdb kad
- Drop now-unused SHA1Comparator
- Efficiency tweak to NodeInfoComparator