MaxBarrierSize of NoOpBarrierStep should be accessible

We would like to use the `maxBarrierSize` parameter of `NoOpBarrierStep`s as a parameter for batch queries in JanusGraph to improve the performance of storage interactions (see JanusGraph Issue 2514).

The purpose of the `NoOpBarrierStep` is to aggregate intermediate results before passing them to the next step in the pipeline. Aligning the size of batch queries with the barrier size helps improving the responsiveness of queries in this scenario.

To do so, we need to access the parameter `maxBarrierSize`. I propose to introduce a public getter method. As of now, the `maxBarrierSize` is already semi-publicly accessible because it is contained in the `toString()` representation of the step.