lttng: make session_storage::_array a unique_ptr to array

By making this field a unique_ptr of lttng_session[] instead of
lttng_session, we can use the subscript operator on it, making the code
in session_list_operations a bit simpler.

Change-Id: Ic4e441a23be834e68c5af3f4cca2794f86f2f57e
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>