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.