Re-add top-level v8::Locker

11d1eca9 added a v8 locker to ease development of 3rd party threading
extensions but it created a condition which would cause node to exit
uncleanly while in debug mode; it was reverted in 7543c38d.

The problem here is that the Locker was being disposed after V8 was torn
down. Adding some scoping fixes that.

src/node.cc

@@ -2644,12 +2644,15 @@ int Start(int argc, char *argv[]) {
   // This needs to run *before* V8::Initialize()
   argv = Init(argc, argv);
 
-  v8::V8::Initialize();
-  v8::HandleScope handle_scope;
+  V8::Initialize();
+  Persistent<Context> context;
+  {
+    Locker locker;
+    HandleScope handle_scope;
 
     // Create the one and only Context.
-  Persistent<v8::Context> context = v8::Context::New();
-  v8::Context::Scope context_scope(context);
+    Persistent<Context> context = Context::New();
+    Context::Scope context_scope(context);
 
     Handle<Object> process_l = SetupProcessObject(argc, argv);
     v8_typed_array::AttachBindings(context->Global());
@@ -2666,10 +2669,13 @@ int Start(int argc, char *argv[]) {
     uv_run(uv_default_loop());
 
     EmitExit(process_l);
+#ifndef NDEBUG
+    context.Dispose();
+#endif
+  }
 
 #ifndef NDEBUG
   // Clean up.
-  context.Dispose();
   V8::Dispose();
 #endif  // NDEBUG