Feature/673 xauth token

### Description
X-Auth-Token forwarded in notifications

Implements #673 
Incoming notifications should be treated the same way as updateContext. In other words, if the notifyContextRequest message includes a X-Auth-Token and that notifyContextRequest triggers a new notification, that notification should include the X-Auth-Token.

To illustrate a bit why this is needed, let's consider the following scenario with 3 federated CBs (CB2 and CB3 are subscribed to the previous CB in the chain):

CB1->CB2->CB3

An updateContext at CB1 including X-Auth-Token should propagate the X-Auth-Token in the chain, i.e. a notification from CB1 to CB2 with X-Auth-Token, that in sequence triggers a new notification from CB2 to CB3 also including the X-Auth-Token.