SessionManager: Ignore Session object destruction during global shutdown

SessionManager: Ignore Session object destruction during global shutdown

We already save all open SessionBackends when shutdown handlers are run,
which *should* make the Session object destructors that run during
global shutdown not have anything to save. But it can get fooled if the
Session data contains other objects that have already gotten destroyed
during the global shutdown, leading to spurious warnings and errors as
it tries to access partically-destroyed objects.

The solution is to set a flag when we do the shutdown handlers and just
ignore the last gasps from Session::__destruct() that might come after.

Change-Id: Ic3eb0bac2d29a30488c84b6525ad796a7f1c9ce9