Build a list of triggers applying to a given session on creation

Build a list of triggers applying to a given session on creation

A hash table associating session names (which are unique) to
a lttng_session_trigger_list object is added to the notification
subsystem. This hash table is populated on the creation of a
session and a list is initialized with matching triggers known
at that time.

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