DSL: Introduce `SubFlow`s

Add `subFlow` support for `router`s
Move some tests to separate domain classes
Fix some component registrations

DSL: Fix `SubFlow` registration according PR discussion

* Add IO plugin
* Fix SI 4.0 <-> 4.1 compatibility by `springIoCheck` results
* Further divide for tests to separate domain classes

DSL: Add `discardFlow` to the `.filter()`

Make `.router()` `subFlow`s as `always return to the main flow`

Add `.publishSubscribeChannel()` EIP-method with `.subscriber()`

to specify subscribers as `subFlow`s

Polishing