Enable to register a processor without running some or all computations

Today it is not possible to use concurrency=0 to disable a computation in a processor setting, this should be fixed.

There are different use cases:

We may want to scale a single or few computations in a processor, this requires to be able to set the computation concurrency to 0 to not create any thread pool on the unwanted computations.

A source stream producer can be in a distinct JVM than the processor, but still, it needs to initialize the processor topology in order to have the source/sink streams initialized (nb of partitions, codec but also filters), this could be done by using a defaultConcurrency=0.