[MEDIUM] stream_interface: added a DISconnected state between CON/EST and CLO

[MEDIUM] stream_interface: added a DISconnected state between CON/EST and CLO

There were rare situations where it was not easy to detect that a failed
session attempt had occurred and needed some server cleanup. In particular,
client aborts sometimes lead to session leaks on the server side.

A new state "SI_ST_DIS" (disconnected) has been introduced for this. When
a session has been closed at a stream interface but the server cleanup has
not occurred, this state is entered instead of CLO. The cleanup is then
performed there and the state goes to CLO.

A new diagram has been added to show possible stream_interface state
transitions that can occur in a stream-sock. It makes debugging easier.