message: [SE] is busy fix
  /build before_merge
diff: @@ -87,6 +87,7 @@ namespace iroha::subscription {
  return true;
  }
  }
  + is_busy_ = false;
  return false;
  }
  @@ -120,13 +121,9 @@ namespace iroha::subscription {
  task();
  } catch (...) {
  }
  - } else {
  - {
  - std::lock_guard lock(tasks_cs_);
  - is_busy_ = false;
  - }
  + } else
  event_.wait(untilFirst());
  - }
  +
  } while (proceed_.test_and_set());
  return 0;
  }
  