relayd: share the same output directory handle accross sessions

Now that lttng_directory_handles are reference counted, they can be
shared by multiple sessions and trace chunks.  These modifications
cause sessions that share a trace chunk to share their session output
directory handle.

Moreover, the session output directory handle is now created on
session creation. Hence, a directory handle (fd) is not created
everytime a trace chunk is created.

The goal of this modification is to make it easier to track file
descriptors in the relay daemon, but it is also more efficient
overall (less opening of file descriptors) and will error-out during
the creation of a session rather than a trace chunk which is handled
more gracefully accross the toolchain.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I47e46ee253db646da1490380612def741c2f4102