changed the way globals behave to be more BC with 1.11 and to avoid speed problems...

changed the way globals behave to be more BC with 1.11 and to avoid speed problems at runtime introduced in 4487387 (closes #932)

* Globals are now managed by themselves, independently of other elements
  contained in extensions

* A global variable value can now be changed after the runtime has been
  initialized (to be more BC with the way Twig 1.11 works)

* Extensions are not initialized anymore when rendering a template that
  is already in the cache (like in Twig 1.11)