nettrace: Add a new tracing library

This commit introduces a new tracing library, that replaces
golang.org/x/net/trace, and supports (amongts other thing) nested
traces.

This is a minimal change, future patches will make use of the new
functionality.

internal/nettrace/evtring.go

@@ -0,0 +1,42 @@
+package nettrace
+
+import "time"
+
+type evtRing struct {
+	evts      []event
+	max       int
+	pos       int // Points to the latest element.
+	firstDrop time.Time
+}
+
+func newEvtRing(n int) *evtRing {
+	return &evtRing{
+		max: n,
+		pos: -1,
+	}
+}
+
+func (r *evtRing) Add(e *event) {
+	if len(r.evts) < r.max {
+		r.evts = append(r.evts, *e)
+		r.pos++
+		return
+	}
+
+	r.pos = (r.pos + 1) % r.max
+
+	// Record the first drop as the time of the first dropped message.
+	if r.firstDrop.IsZero() {
+		r.firstDrop = r.evts[r.pos].When
+	}
+
+	r.evts[r.pos] = *e
+}
+
+func (r *evtRing) Do(f func(e *event)) {
+	for i := 0; i < len(r.evts); i++ {
+		// Go from older to newer by starting at (r.pos+1).
+		pos := (r.pos + 1 + i) % len(r.evts)
+		f(&r.evts[pos])
+	}
+}