tree-wide: install matches asynchronously

Let's remove a number of synchronization points from our service
startups: let's drop synchronous match installation, and let's opt for
asynchronous instead.

Also, let's use sd_bus_match_signal() instead of sd_bus_add_match()
where we can.

src/core/unit.c

@@ -3042,7 +3042,7 @@ int unit_install_bus_match(Unit *u, sd_bus *bus, const char *name) {
                          "member='NameOwnerChanged',"
                          "arg0='", name, "'");
 
-        return sd_bus_add_match(bus, &u->match_bus_slot, match, signal_name_owner_changed, u);
+        return sd_bus_add_match_async(bus, &u->match_bus_slot, match, signal_name_owner_changed, NULL, u);
 }
 
 int unit_watch_bus_name(Unit *u, const char *name) {