Fix: sessiond: don't wait for a rotation from a null chunk to finish

Fix: sessiond: don't wait for a rotation from a null chunk to finish

The rotation completion checking does not handle waiting for the
completion of a rotation _from_ a NULL trace chunk. This is correct as
there is essentially nothing to check. Streams always rotate out of a
null trace chunk immediately as it means the stream had no output and
could not receive data.

Concretely, this happens when stopping a session, rotating, and
re-starting a session.

The fix consists in simply re-working the logic of the "rotate"
command to not launch a rotation completion check job and not put the
session in the "rotation ongoing" state.

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