Acquire a reference to a session when a timer is active

The timers associated to a session don't directly
reference a session since, up to recently, session
objects were not reference counted. There was
essentially no mechanism in place to prevent a
session from being destroyed while one of its timers
was active.

For this reason, the session was queried by id on
every execution of its timers. However, this did
not prevent the session from being destroyed; it only
allowed the periodic jobs to handle that condition
gracefully.

This change that a reference to the session is held
at all times by periodic jobs that are "in-flight".

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