code: @Override
    public long incr(String key, long by, long def) {
      return mutateWithDefault(Mutator.incr, key, by, def, 0);
    }
docs: Increment the given counter, returning the new value.
  
  @param key the key
  @param by the amount to increment
  @param def the default value (if the counter does not exist)
  @return the new value, or -1 if we were unable to increment or add
  @throws OperationTimeoutException if the global operation timeout is
  exceeded
  @throws IllegalStateException in the rare circumstance where queue is too
  full to accept any more requests