Fixed crashes exposed though fuzzing. (Chromium issue 320948)

Deprecated v8::External::New without Isolate parameter.

Made number of available threads isolate-dependent and exposed it to ResourceConstraints. (issue 2991)

Performance and stability improvements on all platforms.

git-svn-id: http://v8.googlecode.com/svn/trunk@17897 ce2b1a6d-e550-0410-aec6-3dcde31c8c00