Split handle scopes into an internal version and a version accessible

Split handle scopes into an internal version and a version accessible
through the API. This allows us to verify state on entry through the API.
In this change verification in the API entry is checking that the current
thread holds the V8 lock when a HandleScope is instantiated if a v8::Locker
has ever been used by the V8 instance.
Review URL: http://codereview.chromium.org/18707

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1140 ce2b1a6d-e550-0410-aec6-3dcde31c8c00