Fix: metadata stream should not reference session

Fix: metadata stream should not reference session

The metadata stream should only reference the metadata cache, not the
session. Otherwise, we end up in a catch 22 situation:

- Stream POLLHUP is only given when the session is destroyed, but,
- The session is only destroyed when all references to session are
  released, including references from channels, but,
- If the metadata stream holds a reference on the metadata session, we
  end up with a circular dependency loop.

Fix this by making sure the metadata stream does not use any of the
lttng channel nor lttng session.

Reviewed-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>