Weld#shutdown() should not fail if the application scope has been invalidated already

Stack trace
http://pastebin.com/3AQDzDSU

Testcase
https://github.com/os890/DS_Discuss/commit/e889ffb55eb96fecae5af6d830bcc685ee4de78a

The task involves

• changing Weld SE's ShutdownManager so that it does not use the CDI scope
• modifying WeldBootstrap#shutdown() to handle already shutdown application scope