Added to mechanism to determine if a client closing is the last one of the session.

Added to mechanism to determine if a client closing is the last one of the session.
Each application creates a file with a lock on it in a subdirectory of the session.
When a client is closing, it checks if there is no other locked file.
If no file, it is the last one and closes the sessions.

git-svn-id: https://svn.lifesci.dundee.ac.uk/svn/repository/trunk@403 be28352e-c001-0410-b1a7-c7978e42abec