Added support for crashed client.

Crashed client are removed correctly from the session client.
When a new client joins a session, it adds itself to the session client list and ensures that it is still in the list while it is active.
When a client wants to remove itself from the session, the client clears the session client list to check if it is the last client. (all actives clients should readd themself to the list during two watcher cycles, if the list is still empty. The client consideres itself as the last client of the session and closes properly the session)

git-svn-id: https://svn.lifesci.dundee.ac.uk/svn/repository/trunk@394 be28352e-c001-0410-b1a7-c7978e42abec