Add Sample Back

The compilation and tests are broken as head.go requires sample which
has been moved to another package while moving BufferedSeriesIterator.

Duplication seemed better compared to exposing sample from tsdbutil.

head.go

@@ -34,6 +34,11 @@ var (
 	ErrOutOfBounds = errors.New("out of bounds")
 )
 
+type sample struct {
+	t int64
+	v float64
+}
+
 // headBlock handles reads and writes of time series data within a time window.
 type headBlock struct {
 	mtx sync.RWMutex