manager: cleanup active connections upon exit

manager: cleanup active connections upon exit

When connection sharing is enabled, the removal of iptables rules is
delegated to the NMActRequest destructor; but for this to work it is
required that the object is properly dereferenced upon NM termination.

Clean up the active connections which are in DEACTIVATED state when
quitting, so that they are unexported and destroyed.

https://bugzilla.gnome.org/show_bug.cgi?id=692673