kv: allocate BatchResponse header and responses together

Avoids a heap allocation on each BatchResponse creations.

```
name                                           old time/op    new time/op    delta
Sysbench/KV/1node_remote/oltp_point_select-10    39.8µs ±16%    39.6µs ±19%    ~     (p=0.968 n=9+10)

name                                           old alloc/op   new alloc/op   delta
Sysbench/KV/1node_remote/oltp_point_select-10    6.79kB ± 4%    6.77kB ± 4%    ~     (p=0.671 n=10+10)

name                                           old allocs/op  new allocs/op  delta
Sysbench/KV/1node_remote/oltp_point_select-10      60.0 ± 0%      59.0 ± 0%  -1.67%  (p=0.000 n=9+8)
```

pkg/kv/kvpb/gen/main.go

@@ -366,13 +366,49 @@ func (ba *BatchRequest) WriteSummary(b *strings.Builder) {
 		allocTypes[resV.variantName] = allocName
 	}
 
+	fmt.Fprint(f, `
+func allocBatchResponse(nResps int) *BatchResponse {
+	if nResps <= 1 {
+		alloc := new(struct {
+			br    BatchResponse
+			resps [1]ResponseUnion
+		})
+		alloc.br.Responses = alloc.resps[:nResps]
+		return &alloc.br
+	} else if nResps <= 2 {
+		alloc := new(struct {
+			br    BatchResponse
+			resps [2]ResponseUnion
+		})
+		alloc.br.Responses = alloc.resps[:nResps]
+		return &alloc.br
+	} else if nResps <= 4 {
+		alloc := new(struct {
+			br    BatchResponse
+			resps [4]ResponseUnion
+		})
+		alloc.br.Responses = alloc.resps[:nResps]
+		return &alloc.br
+	} else if nResps <= 8 {
+		alloc := new(struct {
+			br    BatchResponse
+			resps [8]ResponseUnion
+		})
+		alloc.br.Responses = alloc.resps[:nResps]
+		return &alloc.br
+	}
+	br := &BatchResponse{}
+	br.Responses = make([]ResponseUnion, nResps)
+	return br
+}
+`)
+
 	fmt.Fprint(f, `
 // CreateReply creates replies for each of the contained requests, wrapped in a
 // BatchResponse. The response objects are batch allocated to minimize
 // allocation overhead.
 func (ba *BatchRequest) CreateReply() *BatchResponse {
-	br := &BatchResponse{}
-	br.Responses = make([]ResponseUnion, len(ba.Requests))
+	br := allocBatchResponse(len(ba.Requests))
 
 	counts := ba.getReqCounts()