Consumer perform the rotation when extracting a packet

When the consumer reads a subbuffer, it checks if the stream needs to be
rotated before or after writing the data. The post-rotation action must
take place after we have released the stream lock, so we need to add a
flag to the read_subbuffer functions to know if a rotation occurred
while the stream lock was held.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>