StoppableReadLock(ReadWriteLock lock,CancelCriterion stopper){
  this.lock=lock.readLock();
  this.stopper=stopper;
}

StoppableReadLock ( ReadWriteLock lock , CancelCriterion stopper ) { this . lock = lock . readLock ( ) ; this . stopper = stopper ; }
create a new read lock from the given lock
