Properly handle srcpad events and queries
return EOS if a pad is EOS
Use srcpad.task to create our thread

+ Do not forget to free data in our tests
Allow subclass to easily define the pad type to use for sinkpads
Do no take the lock when fowarding event to sinkpads
Make the pad cond logic per pad
Always protect agg->buffer with the padevent lock
Forward the last _flush_stop after seeking and concider we are still seeking until that happens
Minor doc + debug + naming enhancements
Check whether EOS or FLUSHING *after* making sure the is no buffer on the pad.
What is unlocked may never be unlocked.