@Override
  public OperationFuture<Long> asyncDecr(String key, long by, long def) {
    return asyncMutate(Mutator.decr, key, by, def, 0);
  }
Asynchronous decrement.

@param key key to decrement
@param by the amount to decrement the value by
@param def the default value (if the counter does not exist)
@return a future with the decremented value, or -1 if the decrement failed.
@throws IllegalStateException in the rare circumstance where queue is too
          full to accept any more requests
src/main/java/net/spy/memcached/MemcachedClient.java