v hints, data for non-existing ranges
    * opens multiple channels to the same address
    v hints do not expire
    v RTT calculations need improvement (test)
    v google-log is unnecessary
    * reduce template use (peer queue)
    v hints do not expire
    v survive 10% loss
    * unlimited ping pong
    v git sha-1
    v check hints agains ack_out?_
    v check data against ack_in
    v channel suspend/wake. 3 cong modes state machine - ???
    * release hints for a dormant channel
    * minimize the number of template instantiations
    v Channel thinks how much it HINTs a second,
      picker thinks which HINTs are snubbed
    * files <1sec download : how HINTs are sent?
    * dead Channels are not killed => cannot open a new one
        (have a channel already)
    v peers don't cooperate
    * RecoverProgress fails sometime
    * leecher can't see file is done already
    * why leecher waits 1sec?