tls: destroy SSL once it is out of use

Do not keep SSL structure in memory once socket is closed. This should
lower the memory usage in many cases.

Fix: https://github.com/iojs/io.js/issues/1522
PR-URL: https://github.com/iojs/io.js/pull/1529


Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>