tls: prevent use-after-free

* Destroy `SSL*` and friends on a next tick to make sure that we are not
  doing it in one of the OpenSSL callbacks
* Add more checks to the C++ methods that might be invoked during
  destructor's pending queue cleanup

Fix: https://github.com/joyent/node/issues/8780
Fix: https://github.com/iojs/io.js/issues/1696
PR-URL: https://github.com/nodejs/io.js/pull/1702


Reviewed-By: Trevor Norris <trev.norris@gmail.com>