head: Rename highTimestamp to maxt

`maxt` seems more consistent with `mint` and other uses of `maxt`
elsewhere in the code, if I've understand the intent correctly.

head.go

@@ -449,10 +449,10 @@ func (h *Head) Appender() Appender {
 
 func (h *Head) appender() *headAppender {
 	return &headAppender{
-		head:          h,
-		mint:          h.MaxTime() - h.chunkRange/2,
-		samples:       h.getAppendBuffer(),
-		highTimestamp: math.MinInt64,
+		head:    h,
+		mint:    h.MaxTime() - h.chunkRange/2,
+		maxt:    math.MinInt64,
+		samples: h.getAppendBuffer(),
 	}
 }
 
@@ -469,12 +469,11 @@ func (h *Head) putAppendBuffer(b []RefSample) {
 }
 
 type headAppender struct {
-	head *Head
-	mint int64
+	head       *Head
+	mint, maxt int64
 
-	series        []RefSeries
-	samples       []RefSample
-	highTimestamp int64
+	series  []RefSeries
+	samples []RefSample
 }
 
 func (a *headAppender) Add(lset labels.Labels, t int64, v float64) (uint64, error) {
@@ -508,8 +507,8 @@ func (a *headAppender) AddFast(ref uint64, t int64, v float64) error {
 	if t < a.mint {
 		return ErrOutOfBounds
 	}
-	if t > a.highTimestamp {
-		a.highTimestamp = t
+	if t > a.maxt {
+		a.maxt = t
 	}
 
 	a.samples = append(a.samples, RefSample{
@@ -551,10 +550,10 @@ func (a *headAppender) Commit() error {
 
 	for {
 		ht := a.head.MaxTime()
-		if a.highTimestamp <= ht {
+		if a.maxt <= ht {
 			break
 		}
-		if atomic.CompareAndSwapInt64(&a.head.maxTime, ht, a.highTimestamp) {
+		if atomic.CompareAndSwapInt64(&a.head.maxTime, ht, a.maxt) {
 			break
 		}
 	}