Currently it is possible to run two SAGE notebooks on the same directory,
which is potentially VERY VERY bad. It would be better if when a SAGE
notebook server starts up it checks for the presence of a lock file. This
file would contain the pid of a running SAGE notebook process -- if the file
and that process exist, then the notebook won't start. When the notebook
finishes it should delete that lock file.

Component: notebook

Issue created by migration from https://trac.sagemath.org/ticket/2