Don't keep a thread running in IncrementalArrayData

[BenTilbrook]

IncrementalArrayData keeps a Thread open until it's loaded all increments. This can be a problem if the IncrementalArrayData appears in a nested PowerAdapter structure, because there's no robust way to close the Data instance and stop the Thread.

Perhaps use an ExecutorService instead, such that each increment is a separate task. This would mean the IncrementalArrayData won't strictly need to be closed.