Modifies the behavior of one to one chatroom.

From now on, the conference server NEVER removes participants of a one to one chatroom. The receiving of a BYE is interpreted as a client that wants to disconnect from the chatroom. It will be reINVITEd when needed, when a new message is sent to him.
This greatly simplifies the code and makes it consistent with the fact that participant leaving were not notified.
Additionally, the conference server now adds a header "Session-mode: true" in the SIP MESSAGE so that clients can identify that a message is belonging to a server based chatroom, and report an error when it doesn't have the knowledge about the chatroom.