* qlog: implement a Trace and a Writer struct
* qlog: rename Trace to FileSeq
* split qlog trace writer and QUIC qlog events into separate packages
* use the new qlog.Recorder instead of the logging.ConnectionTracer
* ackhandler: optimize memory layout of packet struct
The packet number can be derived from the position that this packet is
stored at in the packets slice in the sent packet history. There is no
need to store the packet number, saving 8 bytes per packet.
* ackhandler: avoid copying the packet struct
Some of the 10 testing packets are might be lost, while others might be
CE-marked. We need to detect mangling if all testing packets are either
lost are CE-marked.
* ackhandler: detect ECN mangling
Mangling means that a path is re-marking all ECN-marked packets as CE.
* ackhandler: only detect ECN mangling once all testing packets were sent
