crypto: Use reference count to manage cert_store

Setting reference count at the time of setting cert_store instead of
trying to manage it by modifying internal states in destructor.

PR-URL: https://github.com/nodejs/node/pull/9409


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Shigeki Ohtsu <ohtsu@ohtsu.org>