Send filter expression string to session daemon

Send the filter expression string along with the filter's bytecode to
the session daemon. The session daemon now stores the filter expression.
This makes it possible to list the filter's expression when listing
events.

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