@Override
  public OperationFuture<Long> asyncIncr(String key, long by, long def) {
    return asyncMutate(Mutator.incr, key, by, def, 0);
  }
Asychronous increment.

@param key key to increment
@param by the amount to increment the value by
@param def the default value (if the counter does not exist)
@return a future with the incremented value, or -1 if the increment 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