[FLINK-19473] Implement multi inputs sorting DataInput

I implement a MultiInputSortingDataInputs which is kind of a factory for
multiple, related inputs. In case of sorting inputs of Two/Multiple
input operators not only the independent inputs should be sorted, but
also records across different inputs. Therefore the produced inputs
share a common context to synchronize which input should emit records
next.

The coordination is done via inputs Availability. Only one of the
inputs, with the current smallest element, is available at a time.