tp: finish operators after the last input

An operator which holds rows back has nowhere to let them go once the
source runs dry, and no way to report rows it is still holding. Finish()
is called once after the last batch and can return output, fan out, or
fail, just as Execute() can.

The pipeline finishes its operators from the bottom up. Whatever one lets
go is pushed through the operators above it as ordinary input before the
next one is finished, so an operator sees all of its input before its own
Finish() runs.