Improving local dataflow.

  - cleaning up dataflow implementation
  - dataflow is now passing along the futures to allow better error handling
  - adding unwrap utility to allow better code reuse of legacy codes with dataflow, .then and .when style codes
Merge commit '5a83cc7'

Conflicts:
	hpx/lcos/local/dataflow.hpp
	hpx/lcos/local/preprocessed/dataflow_10.hpp
	hpx/lcos/local/preprocessed/dataflow_15.hpp
	hpx/lcos/local/preprocessed/dataflow_20.hpp
	hpx/lcos/local/preprocessed/dataflow_5.hpp