Improves performance of json responses (grafana#7522)

**What this PR does / why we need it**:

This allows to reuse stream memory allocations between responses.
Related grafana#3163 but this time this is
for the encoding.

```
❯ benchstat before.txt after.txt                                                                                                              
name        old time/op    new time/op    delta
_Encode-16    29.2µs ±12%    25.2µs ± 1%  -13.85%  (p=0.016 n=5+4)

name        old alloc/op   new alloc/op   delta
_Encode-16    24.9kB ± 6%    16.4kB ± 8%  -34.20%  (p=0.008 n=5+5)

name        old allocs/op  new allocs/op  delta
_Encode-16       145 ± 0%       129 ± 0%  -11.03%  (p=0.008 n=5+5)
```

pkg/util/marshal/marshal.go

@@ -32,7 +32,11 @@ func WriteQueryResponseJSON(v logqlmodel.Result, w io.Writer) error {
 		},
 	}
 
-	return jsoniter.NewEncoder(w).Encode(q)
+	s := jsoniter.ConfigFastest.BorrowStream(w)
+	defer jsoniter.ConfigFastest.ReturnStream(s)
+	s.WriteVal(q)
+	s.WriteRaw("\n")
+	return s.Flush()
 }
 
 // WriteLabelResponseJSON marshals a logproto.LabelResponse to v1 loghttp JSON
@@ -43,7 +47,11 @@ func WriteLabelResponseJSON(l logproto.LabelResponse, w io.Writer) error {
 		Data:   l.GetValues(),
 	}
 
-	return jsoniter.NewEncoder(w).Encode(v1Response)
+	s := jsoniter.ConfigFastest.BorrowStream(w)
+	defer jsoniter.ConfigFastest.ReturnStream(s)
+	s.WriteVal(v1Response)
+	s.WriteRaw("\n")
+	return s.Flush()
 }
 
 // WebsocketWriter knows how to write message to a websocket connection.
@@ -77,7 +85,11 @@ func WriteSeriesResponseJSON(r logproto.SeriesResponse, w io.Writer) error {
 		adapter.Data = append(adapter.Data, series.GetLabels())
 	}
 
-	return jsoniter.NewEncoder(w).Encode(adapter)
+	s := jsoniter.ConfigFastest.BorrowStream(w)
+	defer jsoniter.ConfigFastest.ReturnStream(s)
+	s.WriteVal(adapter)
+	s.WriteRaw("\n")
+	return s.Flush()
 }
 
 // This struct exists primarily because we can't specify a repeated map in proto v3.
@@ -90,5 +102,9 @@ type seriesResponseAdapter struct {
 // WriteIndexStatsResponseJSON marshals a gatewaypb.Stats to JSON and then
 // writes it to the provided io.Writer.
 func WriteIndexStatsResponseJSON(r *stats.Stats, w io.Writer) error {
-	return jsoniter.NewEncoder(w).Encode(r)
+	s := jsoniter.ConfigFastest.BorrowStream(w)
+	defer jsoniter.ConfigFastest.ReturnStream(s)
+	s.WriteVal(r)
+	s.WriteRaw("\n")
+	return s.Flush()
 }