tls: zero SSL_CTX freelist for a singleUse socket

When connecting to server with `keepAlive` turned off - make sure that
the read/write buffers won't be kept in a single use SSL_CTX instance
after the socket will be destroyed.

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>