Fix: add session_info object to sessions_ht

The notification subsystem currently does not add the session_info
structures it uses to account for session consumed sizes to its
sessions_ht. This means that a new internal session is created
for every channel, resulting in an incorrect accounting of
session consumed sizes, and thus delayed rotations.

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