do_phase_end_session() can also be delayed for some reasons,
depending on applications.  Add GSM_MANAGER_PHASE_EXIT_TIMEOUT
there as well, and drop it to four seconds (meaning that
end session phase and exit phase may together delay logout up to
eight seconds).

The proper fix is to fix the applications, I think.
At least evolution and spotify give these problems.
(see https://bugzilla.redhat.com/show_bug.cgi?id=973613
for discussion regarding evolution).