Command to make a directory on the consumer or relay

This new command allows the session daemon to create a directory in the
session folder (local or remote), this gives more control over the
directory creation which is currently lazy (when the first stream is
created).

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