Add discard channel support for splitters

[alturkovic]

When encountering empty collections, splitter should be able to send the result to a discard channel. Currently, when encountering an empty collection, the splitter ends the flow. Some use-cases may rely on a custom split function which may returns empty collections. These use-cases should be able to define a discard channel so they can proceed with a possible compensation flow.

[artembilan]

Only correction that we should discard a request message, not a result of split function. Something similar what we have with a filter or an aggregator.