[#789] blobstor: Implement iterator

There is a need to be able to process all objects saved in `BlobStor`.

Implement `BlobStor.Iterate` method which iterates over all objects.
Implement `IterateBinaryObjects` and `IterateObjects` helper functions to
simplify the code.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/local_object_storage/blobstor/iterate_test.go

@@ -0,0 +1,92 @@
+package blobstor
+
+import (
+	"encoding/binary"
+	"os"
+	"testing"
+
+	"github.com/nspcc-dev/neofs-api-go/pkg/object"
+	objecttest "github.com/nspcc-dev/neofs-api-go/pkg/object/test"
+	"github.com/nspcc-dev/neofs-node/pkg/local_object_storage/blobovnicza"
+	"github.com/nspcc-dev/neofs-node/pkg/util/logger/test"
+	"github.com/stretchr/testify/require"
+)
+
+func TestIterateObjects(t *testing.T) {
+	p := t.Name()
+
+	const smalSz = 50
+
+	// create BlobStor instance
+	blobStor := New(
+		WithCompressObjects(true, test.NewLogger(false)),
+		WithRootPath(p),
+		WithSmallSizeLimit(smalSz),
+		WithBlobovniczaShallowWidth(1),
+		WithBlobovniczaShallowDepth(1),
+	)
+
+	defer os.RemoveAll(p)
+
+	// open Blobstor
+	require.NoError(t, blobStor.Open())
+
+	// initialize Blobstor
+	require.NoError(t, blobStor.Init())
+
+	defer blobStor.Close()
+
+	const objNum = 5
+
+	type addrData struct {
+		big  bool
+		addr *object.Address
+		data []byte
+	}
+
+	mObjs := make(map[string]addrData)
+
+	for i := uint64(0); i < objNum; i++ {
+		sz := smalSz
+
+		big := i < objNum/2
+		if big {
+			sz++
+		}
+
+		data := make([]byte, sz)
+		binary.BigEndian.PutUint64(data, i)
+
+		addr := objecttest.Address()
+
+		mObjs[string(data)] = addrData{
+			big:  big,
+			addr: addr,
+			data: data,
+		}
+	}
+
+	for _, v := range mObjs {
+		_, err := blobStor.PutRaw(v.addr, v.data)
+		require.NoError(t, err)
+	}
+
+	err := IterateBinaryObjects(blobStor, func(data []byte, blzID *blobovnicza.ID) error {
+		v, ok := mObjs[string(data)]
+		require.True(t, ok)
+
+		require.Equal(t, v.data, data)
+
+		if v.big {
+			require.Nil(t, blzID)
+		} else {
+			require.NotNil(t, blzID)
+		}
+
+		delete(mObjs, string(data))
+
+		return nil
+	})
+	require.NoError(t, err)
+	require.Empty(t, mObjs)
+}