dbus: stash the subscriber list when we disconenct from the bus

dbus: stash the subscriber list when we disconenct from the bus

If we unexpectly disconnect from the bus, systemd would end up dropping
the list of subscribers, which breaks the ability of clients like logind
to monitor the state of units.

Stash the list of subscribers into the deserialized state in the event
of a disconnect so that when we recover we can renew the broken
subscriptions.

(cherry picked from commit 8402ca04d1a063c3d8a9e3d5c16df8bb8778ae98)