Hardcoded initial delay for completionInterval in AggregateProcessor

When setting the completionInterval on an aggregator, the first completion will be triggered after 1000ms. The reason is the hardcoded initial delay in AggregateProcessor.

This behavior has not been documented and is not expected. The expected behavior is for the first completion to be triggered after the given interval. The first completion would then be triggered after completionInterval, the second after 2 * completionInterval, the third after 3 * completionInterval and so on.