session: add session eventing infra for apps

session: add session eventing infra for apps

Basic framework/supporting infrastructure for now.

Start evt app with default collector with:
 app evt-collector enable [uri <uri>]

Add/del collectors (only one supported for now) with:
 app evt-collector add <uri>

External applications can request eventing to default collector. Builtin
applications can also provide custom eventing functions.

Type: feature

Change-Id: I3547bfc9b258b33a4e8c60c161de75c21533b7f1
Signed-off-by: Florin Coras <fcoras@cisco.com>