introduce a ConnectionTracingID type for the ConnectionTracingKey (#4400)

This is a breaking API change. Unfortunately, context values are not
strongly typed, so users will have have to take to change their type
assertions.

client.go

@@ -35,7 +35,7 @@ type client struct {
 	conn quicConn
 
 	tracer    *logging.ConnectionTracer
-	tracingID uint64
+	tracingID ConnectionTracingID
 	logger    utils.Logger
 }