Don't apply default route until zerotier is "online"

I was running into issues with restarting the zerotier service while
"full tunnel" mode is enabled.
When zerotier first boots, it gets network state from the cache
on disk. So it immediately applies all the routes it knew about
before it shutdown.
The network config may have change in this time.
If it has, then your default route is via a route
you are blocked from talking on. So you  can't get the current
network config, so your internet does not work.

Other options include
- don't use cached network state on boot
- find a better criteria than "online"

service/OneService.cpp

@@ -2617,8 +2617,9 @@ public:
 					r->second->sync();
 			}
 			for(std::map< InetAddress, SharedPtr<ManagedRoute> >::iterator r(n.managedRoutes().begin());r!=n.managedRoutes().end();++r) {
-				if (r->second->via())
+				if (r->second->via() && (!r->second->target().isDefaultRoute() || _node->online())) {
 					r->second->sync();
+				}
 			}
 		}