If a server closes the XMPP stream, it sends a </stream:stream> tag. The client is supposed to respond with the same tag and then both parties can close the TLS/TCP socket. Previously Swift(-en) would simply ignore </stream:stream> tag if it was not directly followed by a shutdown of the TCP connection. In addition there is now a timeout timer started as soon as Swiften or the server initiates a shutdown. It will close the socket and cleanup the ClientSession if the server does not respond in time or the network is faulty. Refactored some code in ClientSession in the process. Moved ClientSession::State to a C++11 strongly typed enum class. This also fixes issues where duplicated </stream:stream> tags would be send by Swift. Test-Information: Tested against Prosody ba782a093b14 and M-Link 16.3v6-0, which provide ad-hoc commands to end a user session. Previously this was ignored by Swift. Now it correctly responds to the server, detects it as a disconnect and tries to reconnect afterwards. Added unit test for the case where the server closes the session stream. Change-Id: I59dfde3aa6b50dc117f340e5db6b9e58b54b3c60