lok preinit: don't re-synchronize extensions on each child start.

We should sort out our extensions, if at all, at pre-init time.
This should save ~4% of CPU in a week-long profile of a COOL server.
From Desktop::Main's SynchronizeExtensionRepositories.

Change-Id: I2760de26635723492dfa5a52005ee90623337bee
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/165622
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolan.mcnamara@collabora.com>

desktop/inc/app.hxx

@@ -69,7 +69,7 @@ class Desktop final : public Application
 
                                 Desktop();
                                 virtual ~Desktop() override;
-        virtual int             Main( ) override;
+        virtual int             Main() override;
         virtual void            Init() override;
         virtual void            InitFinished() override;
         virtual void            DeInit() override;

desktop/source/lib/init.cxx

@@ -7509,6 +7509,9 @@ static void preloadData()
     if(bAbort)
         std::cerr << "CheckExtensionDependencies failed" << std::endl;
 
+    // inhibit forced 2nd synchronization from Main
+    ::rtl::Bootstrap::set( "DISABLE_EXTENSION_SYNCHRONIZATION", "true");
+
     std::cerr << "Preload textencodings"; // sal_textenc
     // Use RTL_TEXTENCODING_MS_1250 to trigger Impl_getTextEncodingData
     // to dlopen sal_textenclo