Allow detection of session closure

In some case it can be good to have a function called
when the session closes. This is the case when a binding
records an item in the context. With this modification
the recorded item can be simply NULL.

Change-Id: I5e23f00eb2a6e2639fdeea15f9dea79c7df9dfbb
Signed-off-by: José Bollo <jose.bollo@iot.bzh>