crypto: store thread id as pointer-sized value

uv_thread_t is a HANDLE (void pointer) on Windows, which means that
on 64-bit windows it cannot be stored with CRYPTO_THREADID_set_numeric
without potential data loss.

PR-URL: https://github.com/iojs/io.js/pull/124


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>