Change trace_path to session_root_path and chunk_path

Prepare for the trace rotation feature where we need to store the root
path of the session and create a subdirectory for each chunk of trace.
For now, the chunk_path is \0, so the behaviour does not change.

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