sessiond: notification: maintain an id to notification_client ht

In preparation for the addition of an action execution worker, add a
client_id_ht which will allow the action worker to send commands to
the notification thread referencing clients by 'id' rather than by
their socket.

This is done in order to prevent FD re-use races between the various
threads at play when a communication error occurs on a notification
client socket.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I7b8774c23a4a2a7c41ed8c1652f23d29954a0771