Implement BBR start-up properly

[huitema]

The current BBR code switches to using Hystart if the delay is long enough. In theory, it should not have too, but in practice:

• Hystart exits slow start sooner, as soon as delay increases. BBR only exits if the bottleneck rate did not grow for four periods, which can cause overloading of the network. This means avoiding packet losses more often with Hystart than with BBR.
• Hystarts support external input like data rate, which allows growing CWIN faster

This should be fixed

[huitema]

Tests show that this probably a bad idea. The BBR startup design works as a succession of "epochs" and extends for another 3 RTT after the maximum bandwidth has been found. This seems to be inferior to the HyStart approach in most cases.