objects: per-object tree lock

This replaces the structure lock per instance with a lock per (parent)
object. The point is to reduce lock contention in vlc_object_release().

src/libvlc.h

@@ -155,9 +155,6 @@ typedef struct libvlc_priv_t
     struct playlist_preparser_t *parser; ///< Input item meta data handler
     struct vlc_actions *actions; ///< Hotkeys handler
 
-    /* Objects tree */
-    vlc_mutex_t        structure_lock;
-
     /* Exit callback */
     vlc_exit_t       exit;
 } libvlc_priv_t;

src/misc/variables.h

@@ -47,6 +47,7 @@ struct vlc_object_internals
     vlc_object_internals_t *next;  /* next sibling */
     vlc_object_internals_t *prev;  /* previous sibling */
     vlc_object_internals_t *first; /* first child */
+    vlc_mutex_t tree_lock;
 };
 
 # define vlc_internals( obj ) (((vlc_object_internals_t*)(VLC_OBJECT(obj)))-1)