Data format event listners listens to different events a data format handler
can raise during data access; e.g. end of data etc. Listners can be
registered for different data format handlers if they support it.
A data format handler can support streaming fetch. I.e. the data can only be
fetched on demand from the common ADF runtime. Listeners can be useful tools
to maintain connections etc. that needs to know the state of data access.