Make composite cookers lazily evaluated (e.g. do not call OnDataAvailable during processing, but instead the first time that cooker is queried)

[mslukebo]

Is your feature request an entirely new concept? New concept: make composite cookers lazily evaluated

Is your feature request related to an existing component? Yes - composite data cookers.

Is your feature request related to a problem? Please describe. When processing a data source, processing time is currently coupled with the number of data cookers involved in the data processing pipeline it starts. It's possible, for instance, to break a trace's processing by creating a cooker that never returns from OnDataAvailable.

Describe the solution you'd like Instead of calling OnDataAvailable during processing, we should defer its call until the first time a consumer queries that cooker for an output.