Create an initial trace chunk on first start of an ltt_session

Add a current trace chunk to an ltt_session on session start.
The session base output directory is created at that moment and an
handle to it is provided to the session daemon's trace chunk when
it is set in 'owner' mode.

Setting the trace chunk in "owner" mode results in the creation of
the chunk's output directory. In the case, of the first chunk,
the chunk output directory matches the session output directory.
Hence, this has no visible effect.

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