[#1204] shard: Save ID in the metabase

`AddShard` must return shard id, so we temporarily open metabase
there.

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

pkg/local_object_storage/metabase/shard_id.go

@@ -0,0 +1,36 @@
+package meta
+
+import (
+	"github.com/nspcc-dev/neo-go/pkg/util/slice"
+	"go.etcd.io/bbolt"
+)
+
+var (
+	shardInfoBucket = []byte(invalidBase58String + "i")
+	shardIDKey      = []byte("id")
+)
+
+// ReadShardID reads shard id from db.
+// If id is missing, returns nil, nil.
+func (db *DB) ReadShardID() ([]byte, error) {
+	var id []byte
+	err := db.boltDB.View(func(tx *bbolt.Tx) error {
+		b := tx.Bucket(shardInfoBucket)
+		if b != nil {
+			id = slice.Copy(b.Get(shardIDKey))
+		}
+		return nil
+	})
+	return id, err
+}
+
+// WriteShardID writes shard it to db.
+func (db *DB) WriteShardID(id []byte) error {
+	return db.boltDB.Update(func(tx *bbolt.Tx) error {
+		b, err := tx.CreateBucketIfNotExists(shardInfoBucket)
+		if err != nil {
+			return err
+		}
+		return b.Put(shardIDKey, id)
+	})
+}