[#1324] local_object_storage: Implement tree service backend

In this commit we implement algorithm for CRDT trees from
https://martin.klepmann.com/papers/move-op.pdf

Each tree is identified by the ID of a container it belongs to
and the tree name itself. Essentially, it is a sequence of operations
which should be applied in chronological order to get a usual tree
representation.

There are 2 backends for now: bbolt database and in-memory.
In-memory backend is here for debugging and will eventually act
as a memory-cache for the on-disk database.

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

pkg/local_object_storage/pilorama/meta_test.go

@@ -0,0 +1,54 @@
+package pilorama
+
+import (
+	"math/rand"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestMeta_Bytes(t *testing.T) {
+	t.Run("empty", func(t *testing.T) {
+		var m Meta
+		require.NoError(t, m.FromBytes(nil))
+		require.True(t, len(m.Items) == 0)
+		require.Equal(t, uint64(0), m.Time)
+		require.Equal(t, []byte{0, 0}, m.Bytes())
+	})
+	t.Run("filled", func(t *testing.T) {
+		expected := Meta{
+			Time: 123,
+			Items: []KeyValue{
+				{"abc", []byte{1, 2, 3}},
+				{"xyz", []byte{5, 6, 7, 8}},
+			}}
+
+		data := expected.Bytes()
+
+		var actual Meta
+		require.NoError(t, actual.FromBytes(data))
+		require.Equal(t, expected, actual)
+
+		t.Run("error", func(t *testing.T) {
+			require.Error(t, new(Meta).FromBytes(data[:len(data)/2]))
+		})
+	})
+}
+
+func TestMeta_GetAttr(t *testing.T) {
+	attr := [][]byte{
+		make([]byte, 5),
+		make([]byte, 10),
+	}
+	for i := range attr {
+		rand.Read(attr[i])
+	}
+
+	m := Meta{Items: []KeyValue{{"abc", attr[0]}, {"xyz", attr[1]}}}
+	require.Equal(t, attr[0], m.GetAttr("abc"))
+	require.Equal(t, attr[1], m.GetAttr("xyz"))
+
+	require.Nil(t, m.GetAttr("a"))
+	require.Nil(t, m.GetAttr("xyza"))
+	require.Nil(t, m.GetAttr(""))
+}