Add an atexit handler to remove the D-Bus socket

We should try not to leave stale sockets and their directories when exiting.
They are removed when calling atk_bridge_adaptor_cleanup, but gtk does not
currently call this function when exiting, and there is not a good place
to call it from gtk.

https://bugzilla.gnome.org/show_bug.cgi?id=684076
Only send cache-related events for transients when explicitly requested

Hopefully this will improve performance when, ie, gtk floods us with
children-changed events when populating a large treeview.

https://bugzilla.gnome.org/show_bug.cgi?id=728319
2.13.1