Tweaks around handling of --limit-rate:

* tracing: trace outstanding timeouts by name
* multi: do not mark transfer as dirty that have
  an EXPIRE_TOOFAST set
* multi: have one static function to asses speed limits
* multi: when setting EXPIRE_TOOFAST remove the transfers
  from the dirty set
* progress: rename vars and comment on how speed limit
  timeouts are calculated, for clarity
* transfer: when speed limiting, exit the receive loop
  after a quarter of the limit has been received, not
  on the first chunk received.
* cf-ip-happy.c: clear EXPIRE_HAPPY_EYEBALLS on connect
* scorecard: add --limit-rate parameter to test with
  speed limits in effect