* Netdb exploration:

    - Remove floodfills from dont-include list in exploration
      DatabaseLookupMessage, as the dont-include-floodfills flag
      has been supported since release 0.7.9.
      This will make exploration work better, as there is room for
      non-floodfills in the don't-include list now.
    - Reduce min and max exploration intervals
    - Explore aggressively at startup and if known routers is low
    - Explore slowly if known routers is high