Allow to set the context for which sessions can be used.

Motivation:

Openssl supports the SSL_CTX_set_session_id_context function to limit for which context a session can be used. We should support this.

Modifications:

Add OpenSslServerSessionContext that exposes a setSessionIdContext(...) method now.

Result:

It's now possible to use SSL_CTX_set_session_id_context.