GroupByTime should issue OnCompleted to each group before moving to next chunk

[glopesdev]

The current default behavior of GroupByTime when no clock source is provided is to apply a simple GroupBy on the input stream using the computed chunk index as key. This will leave the created groups alive since GroupBy has no way of knowing if other messages with the same chunk index will arrive later.

Ideally, each chunk group should be closed with OnCompleted as soon as a new group is created. We need to think about how to deal with possible resurrections of old groups too, in cases where clock is moved backwards, e.g. when connecting a new synchronizer, etc.