Flattens the given flow of flows into a single flow with a concurrency limit on the number of concurrently collected flows.
If concurrency is more than 1, then inner flows are be collected by this operator concurrently. With
concurrency == 1 this operator is identical to flattenConcat.
Applications of flowOn, buffer, and produceInafter this operator are fused with its concurrent merging so that only one properly configured channel is used for execution of merging logic.
When concurrency is greater than 1, this operator is buffered by default and size of its output buffer can be changed by applying subsequent buffer operator.