Function _check_abandoned

include/process/check.hpp:108–120

template <typename T>
Option<Error> _check_abandoned(const process::Future<T>& f)
{
  if (f.isReady()) {
    return Some("is READY");
  } else if (f.isDiscarded()) {
    return Some("is DISCARDED");
  } else if (f.isFailed()) {
    return Some("is FAILED: " + f.failure());
  } else if (!f.isAbandoned()) {
    return Some("is not abandoned");
  }
  return None();
}

// TODO(dhamon): CHECK_NPENDING, CHECK_NREADY, etc.