Don't use TLS for space iterators.

This is not only inherently slow, but it also forces the caller to enter an
Isolate before. Both is bad, so we have to do some heap plumbing.

BUG=v8:2531

Review URL: https://codereview.chromium.org/12217106

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13638 ce2b1a6d-e550-0410-aec6-3dcde31c8c00