Mir cannot restart after a crash because /tmp/mir_socket is not deleted

[Eleni Maria Stea]

When mir crashes or exits abnormally /tmp/mir_socket is not deleted. Next time I try to start mir (I tried using "start lightdm") I get the following error in /var/log/lightdm/unity-system-compositor.log:

Dynamic exception type: boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<boost::system::system_error> >
std::exception::what: bind: Address already in use

$ ps aux | grep mir
eleni 12421 0.0 0.0 13644 964 pts/9 S+ 16:58 0:00 grep --color=auto mir
shows that xmir is not running.

If I manually delete /tmp/mir_socket, the problem is fixed and in next lightdm start or restart mir is up again.