stream: Correct Transform class backpressure

The refactor in b43e544140ccf68580c02e71c56d19b82e1e1d32 to use
stream.push() in Transform inadvertently caused it to immediately
consume all the written data, regardless of whether or not the readable
side was being consumed.

Only pull data through the _transform() process when the readable side
is being consumed.

Fix #4667