Fix: session conditions not evaluated at subscription/registration

Conditions bound to sessions (session rotation ongoing/completed)
are not automatically evaluated when a notification channel client
subscribes to them or when a client is subscribed _before_ the
trigger is created.

The problematic scenario is:

- Trigger is registered to notify on session rotation ongoing for
  session 'foo',
- A rotation is launched on session foo (but not completed)
- A client subscribes to 'session rotation ongoing' notifications for
  session 'foo'

In this scenario, the client would not be notified of the 'current'
state of the session.

Whether or not a client is notified of the 'current' state at the
time of subscription/registration is defined per-condition. In
the case of 'session rotation ongoing', it is desirable for clients
to be notified that the rotation is ongoing at the time of their
subscription/registration.

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