session: allow transports to generate closed notifications

session: allow transports to generate closed notifications

In contrast to the closing notification, whereby a transport informs
the session layer that is beginning the closing procedure, this allows
transports to notify the session layer of the fact that the transport is
"fully" closed, i.e., it expects no more data.

Also:
- adds app closed state for sessions
- changes tcp to have it notify when an active close has finished

Type: feature

Change-Id: I13c738006c03f85015e05ab82843a33a69382aaf
Signed-off-by: Florin Coras <fcoras@cisco.com>