* Allow the streaming lib resend frequency to drop down to 20s as the
      minimum, so that up to 2 retries can get sent on an http request.
    * Add further limits to failsafe tunnels.
    * Keep exploratory and client tunnel testing and building stats separate.
    * Only use the 60s period for throttling tunnel requests due to transient
      network overload.
    * Rebuild tunnels earlier (1-3m before expiration, by default)
    * Cache the next hop's routerInfo for participating tunnels so that the
      tunnel participation doesn't depend on the netDb.
    * Fixed a long standing bug in the streaming lib where we wouldn't always
      unchoke messages when the window size grows.
    * Make sure the window size never reaches 0 (duh)