Allow to write a custom verification callback

Motivation:

To be able to hook in TrustManager into the handshake verification we need a way to set a callback.

Modifications:

Add SSLContext.setCertVerifyCallback(...) which allows to set a callback which will do the whole cerification verification and so replace the default of openssl by using SSL_CTX_set_cert_verify_callback(...)

Result:

It's possible to write a custom verification callback.