Extend SessionState to track whether a Logout has been received

If the onLogout() callback is executed, it would be nice to know whether this was a normal logout,
ie the other side sent a "Logout" (and we responded to it), or whether we sent a Logout and got an answer.

I suggest to add the boolean variable receivedLogout with appropriate setter and getter methods to the SessionState:

public boolean isLogoutReceived()
public void setLogoutReceived(boolean logoutReceived)

The flag is cleared initially and set in nextLogout() to indicate that a Logout has been received (or sent). In the method
disconnect(), the flag is cleared after the call to the onLogout() callback.

A delegate method isLogoutReceived() is added to the Session object to make the current state accessible from outside.

Example usage:

public void onLogout(SessionID sessionID)

{ String disconnectDescription = ( session.receivedLogout() ? "Regular" : "Unsolicited" ); ... }