Fix bbr startup

[huitema]

The current implementation of BBR replaces the "startup" code of BBR by Hystart when the latency is above a threshold. This was due to two perceived issues with the BBR startup:

• it did not exit if detecting an increase in RTT, which could lead to high losses if the path buffer were small
• it did not properly incorporate the link rate measurements, which leads to slow startup on long delay links.

[huitema]

Abandoning this PR. There does not seem to be a good way to make BBR startup performance match HyStart, probably because testing on a succession of epochs is too coarse compared to testing the RTT of every packet.