Add specs for Follower#lag

spec/replication/follower_spec.cr

@@ -144,4 +144,28 @@ module FollowerSpec
       file_list.should eq file_index.@files_with_hash
     end
   end
+
+  describe "#lag" do
+    it "should count bytes added to action queue" do
+      follower_socket, _client_socket = FakeSocket.pair
+      file_index = FakeFileIndex.new
+      follower = LavinMQ::Replication::Follower.new(follower_socket, file_index)
+      filename = "#{LavinMQ::Config.instance.data_dir}/file1"
+      size = follower.append filename, Bytes.new(10)
+      follower.lag.should eq size
+    end
+
+    it "should subtract acked bytes from lag" do
+      follower_socket, client_socket = FakeSocket.pair
+      file_index = FakeFileIndex.new
+      follower = LavinMQ::Replication::Follower.new(follower_socket, file_index)
+      filename = "#{LavinMQ::Config.instance.data_dir}/file1"
+      size = follower.append filename, Bytes.new(10)
+      size2 = follower.append filename, Bytes.new(20)
+      # send ack for first message
+      client_socket.write_bytes size.to_i64, IO::ByteFormat::LittleEndian
+      follower.read_ack
+      follower.lag.should eq size2
+    end
+  end
 end