Add stateful mapping

[je-ik]

Sometimes (specifically on inputs) it makes a lot of sense to enable stateful processing without additional shuffle. This is most valuable on inputs, because after the input, partitioning is no longer defined or guaranteed. We will enhance FlatMap operator, that will take a StatefulUnaryFunctor, which will be sort of rich function with enhanced life-cycle methods (setup(Context), cleanup(Context)) and the apply method will take StatefulContext, with access to StorageProvider. We will consider possibility to add this special version of flatmap operator only attached to inputs. This is optional, though.

[je-ik]

Inputs are handled by beam IOs and we have found no useful application of stateful mapping.