Replace synchronization semaphore with async messages

* Remove SyncToServices, I will probably readd it in future
but this time using a local synchronization service more than
relying on the media_server to release the semaphore.
* Due to some discussions today in mailing list I decided
to step back and retry the initial way to notify media rosters
about media services status. It is woking by using two different
notifications for reconnecting to the media_server and notifying
the local subscribers.
* This speed up the media services restart.
* Sorry for the noise and very thanks for reviewing my code to
everyone.

headers/os/media/MediaRoster.h

@@ -44,10 +44,6 @@ public:
 	// Check if the media services are running.
 	static	bool				IsRunning();
 
-	// This functions blocks until the media services are available,
-	// don't abuse of it.
-	static	status_t			SyncToServices(bigtime_t timeout = -1);
-
 	// Getting common instances of system nodes:
 			status_t			GetVideoInput(media_node* _node);
 			status_t			GetAudioInput(media_node* _node);

headers/private/media/ServerInterface.h

@@ -30,6 +30,8 @@ enum {
 
 	// sent by the rescan thread
 	MEDIA_SERVER_RESCAN_COMPLETED,
+	// sent to rosters when rescan is completed
+	MEDIA_SERVER_ALIVE,
 
 	// media add-on server
 	MEDIA_ADD_ON_SERVER_PLAY_MEDIA = '_TRU'
@@ -373,7 +375,6 @@ struct server_register_app_request : request_data {
 };
 
 struct server_register_app_reply : reply_data {
-	sem_id					global_synchro;
 };
 
 struct server_unregister_app_request : request_data {