bus: introduce concept of a "default" event loop per-thread and make use of it everywhere

Try to emphasize a bit that there should be a mapping between event
loops and threads, hence introduce a logic that there's one "default"
event loop for each thread, that can be queried via
"sd_event_default()".

src/network/networkd-manager.c

@@ -31,7 +31,7 @@ int manager_new(Manager **ret) {
         if (!m)
                 return -ENOMEM;
 
-        r = sd_event_new(&m->event);
+        r = sd_event_default(&m->event);
         if (r < 0)
                 return r;