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.
actually, do we really need this exception message? It looks to me that the type of the exception tells the same information already. Same for all other `throw new SslPeerUnverifiedException(...)` stmts everywhere.