service: Use sync pool for Sign/Verify request headers

```
// Before
BenchmarkSignRequestHeader-8   	     146	   8070375 ns/op	 4210607 B/op	      48 allocs/op
BenchmarkVerifyRequestHeader-8   	      14	  83058325 ns/op	42085955 B/op	    1601 allocs/op

// After
BenchmarkSignRequestHeader-8   	     156	   7709172 ns/op	   33902 B/op	      45 allocs/op
BenchmarkVerifyRequestHeader-8   	      15	  76910232 ns/op	   54368 B/op	    1563 allocs/op

// Summary:
benchmark                          old ns/op     new ns/op     delta
BenchmarkSignRequestHeader-8       8070375       7709172       -4.48%
BenchmarkVerifyRequestHeader-8     83058325      76910232      -7.40%

benchmark                          old allocs     new allocs     delta
BenchmarkSignRequestHeader-8       48             45             -6.25%
BenchmarkVerifyRequestHeader-8     1601           1563           -2.37%

benchmark                          old bytes     new bytes     delta
BenchmarkSignRequestHeader-8       4210607       33902         -99.19%
BenchmarkVerifyRequestHeader-8     42085955      54368         -99.87%
```

go.mod

@@ -18,3 +18,6 @@ require (
 	golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550
 	google.golang.org/grpc v1.24.0
 )
+
+// Used for debug reasons
+// replace github.com/nspcc-dev/neofs-crypto => ../neofs-crypto

service/verify.go

@@ -2,6 +2,7 @@ package service
 
 import (
 	"crypto/ecdsa"
+	"sync"
 
 	crypto "github.com/nspcc-dev/neofs-crypto"
 	"github.com/nspcc-dev/neofs-proto/internal"
@@ -12,7 +13,8 @@ import (
 type (
 	// VerifiableRequest adds possibility to sign and verify request header.
 	VerifiableRequest interface {
-		Marshal() ([]byte, error)
+		Size() int
+		MarshalTo([]byte) (int, error)
 		AddSignature(*RequestVerificationHeader_Signature)
 		GetSignatures() []*RequestVerificationHeader_Signature
 		SetSignatures([]*RequestVerificationHeader_Signature)
@@ -133,6 +135,10 @@ func newSignature(key *ecdsa.PrivateKey, data []byte) (*RequestVerificationHeade
 	}, nil
 }
 
+var bytesPool = sync.Pool{New: func() interface{} {
+	return make([]byte, 4.5*1024*1024) // 4.5MB
+}}
+
 // SignRequestHeader receives private key and request with RequestVerificationHeader,
 // tries to marshal and sign request with passed PrivateKey, after that adds
 // new signature to headers. If something went wrong, returns error.
@@ -146,12 +152,23 @@ func SignRequestHeader(key *ecdsa.PrivateKey, msg VerifiableRequest) error {
 		}()
 	}
 
-	data, err := msg.Marshal()
+	data := bytesPool.Get().([]byte)
+	defer func() {
+		bytesPool.Put(data)
+	}()
+
+	if size := msg.Size(); size <= cap(data) {
+		data = data[:size]
+	} else {
+		data = make([]byte, size)
+	}
+
+	size, err := msg.MarshalTo(data)
 	if err != nil {
 		return err
 	}
 
-	signature, err := newSignature(key, data)
+	signature, err := newSignature(key, data[:size])
 	if err != nil {
 		return err
 	}
@@ -174,8 +191,10 @@ func VerifyRequestHeader(msg VerifiableRequest) error {
 		}()
 	}
 
+	data := bytesPool.Get().([]byte)
 	signatures := msg.GetSignatures()
 	defer func() {
+		bytesPool.Put(data)
 		msg.SetSignatures(signatures)
 	}()
 
@@ -189,9 +208,15 @@ func VerifyRequestHeader(msg VerifiableRequest) error {
 			return errors.Wrapf(ErrCannotLoadPublicKey, "%d: %02x", i, peer)
 		}
 
-		if data, err := msg.Marshal(); err != nil {
+		if size := msg.Size(); size <= cap(data) {
+			data = data[:size]
+		} else {
+			data = make([]byte, size)
+		}
+
+		if size, err := msg.MarshalTo(data); err != nil {
 			return errors.Wrapf(err, "%d: %02x", i, peer)
-		} else if err := crypto.Verify(key, data, sign); err != nil {
+		} else if err := crypto.Verify(key, data[:size], sign); err != nil {
 			return errors.Wrapf(err, "%d: %02x", i, peer)
 		}
 	}

service/verify_test.go

@@ -14,6 +14,57 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func BenchmarkSignRequestHeader(b *testing.B) {
+	key := test.DecodeKey(0)
+
+	custom := testCustomField{1, 2, 3, 4, 5, 6, 7, 8}
+
+	some := &TestRequest{
+		IntField:    math.MaxInt32,
+		StringField: "TestRequestStringField",
+		BytesField:  make([]byte, 1<<22),
+		CustomField: &custom,
+		RequestMetaHeader: RequestMetaHeader{
+			TTL:   math.MaxInt32 - 8,
+			Epoch: math.MaxInt64 - 12,
+		},
+	}
+
+	b.ResetTimer()
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		require.NoError(b, SignRequestHeader(key, some))
+	}
+}
+
+func BenchmarkVerifyRequestHeader(b *testing.B) {
+	custom := testCustomField{1, 2, 3, 4, 5, 6, 7, 8}
+
+	some := &TestRequest{
+		IntField:    math.MaxInt32,
+		StringField: "TestRequestStringField",
+		BytesField:  make([]byte, 1<<22),
+		CustomField: &custom,
+		RequestMetaHeader: RequestMetaHeader{
+			TTL:   math.MaxInt32 - 8,
+			Epoch: math.MaxInt64 - 12,
+		},
+	}
+
+	for i := 0; i < 10; i++ {
+		key := test.DecodeKey(i)
+		require.NoError(b, SignRequestHeader(key, some))
+	}
+
+	b.ResetTimer()
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		require.NoError(b, VerifyRequestHeader(some))
+	}
+}
+
 func TestSignRequestHeader(t *testing.T) {
 	req := &TestRequest{
 		IntField:    math.MaxInt32,