* qlog: split the PTO count updates ouf of the MetricsUpdated event
This allows us to not use pointers for all member variables of both
events.
* ackhandler: avoid spurious call to GetCongestionWindow
* utils: initialize the {Smoothed, Latest, Min}RTT to 100ms
* utils: use time.Duration.Nanoseconds instead of uint64
No functional change expected.
* congestion: better check to avoid division by zero
* 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
