Add partitioned transactional source phase 2

[JustinPihony]

Short description

Complete the partitioned transactional source work started in #930

Details

As noted in the first PR:

Test: Consistency of partitioned source when calling shutdown using draining control
Set ConsumerSettings.stopTimeout to 0 to allow for faster shutdowns when a stream completes/fails.
Update docs. (to be continued in phase 2)
Add more low level testing of draining control logic