Patch by Dmitry Lomov.

pthreads implementations are free to reuse pthread_t (thread id) after
the thread has died. This change gets rid of ThreadHandle class and
replaces it with v8-managed thread identifiers.


git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7575 ce2b1a6d-e550-0410-aec6-3dcde31c8c00