Add test that ensures the committing happens correctly inside Headers Stage

[gakonst]

We should have a test that blocks the stream per-batch, letting us check that the commit actually happened while the stage is running. Currently we're only testing that the entire stream has been committed.

Originally posted by @gakonst in #406 (comment)

[MegaRedHand]

I think this issue can be closed. After #609, the headers stage doesn't commit directly, and returns control to the pipeline after a number of headers. Although we are missing some test that tries to execute the stage in multiple steps (see BodyStage's sync_from_previous_progress).