Users are sometimes logged in with another user account (tomcat 8)

Hi,

It happens that users are suddenly logged in with another user account.

As discussed on IRC, the issue could have a link with XWIKI-4109 and tomcat 8 (which is the container we are running).

The issue is reproduced in the following video : https://box.ow2.org/public.php?service=files&t=8fec919e43abff5229d3423360124571