Replace RowOperation.initialValue() and rowAggregator() by collect(Collector)

[Douglas Surber]

> On Oct 25, 2017, at 8:40 AM, Lukas Eder <lukas.eder at gmail.com> wrote:
> 
> In my suggestion, I assumed that OperationGroup.rowOperation could return a RowOperation<?>, and only calling the RowOperation.collect() method would make the type parameter more concrete, thus collect() would have to be a generic method. All the other methods on RowOperation currently do not rely on its type parameter.
> 

submit does and by transitivity toCompletableFuture, which is where the real problems show up.

Douglas