Add OpenSslClientContext to allow creating SslEngine for client side

Motivation:

We only support openssl for server side at the moment but it would be also useful for client side.

Modification:

* Upgrade to new netty-tcnative snapshot to support client side openssl support
* Add OpenSslClientContext which can be used to create SslEngine for client side usage
* Factor out common logic between OpenSslClientContext and OpenSslServerContent into new abstract base class called OpenSslContext
* Correctly detect handshake failures as soon as possible
* Guard against segfault caused by multiple calls to destroyPools(). This can happen if OpenSslContext throws an exception in the constructor and the finalize() method is called later during GC

Result:

openssl can be used for client and servers now.
It's an overridden method, so we don't need to write Javadoc here. (and you copy-and-pasted it, dude!)