SNI support  rfc-6066

Add SNI support rfc-6066.
Example of usage (you need need to implement  tlsCtxStorage object):

```go
// callback
func sniCallback(ssl *openssl.SSL) openssl.SSLTLSExtErr {
	host  := ssl.GetServername()

	if len(host) == 0 {
		return openssl.SSLTLSEXTErrNoAck
	}

	// check if we are already in default host
	if defaulHost, err := tlsCtxStorage.GetDefaultHost(); host == defaulHost && 
		err == nil {
			return openssl.SSLTLSExtErrOK
	}

	// get ctx
	ctx, err := tlsCtxStorage.GetCtx(host);
	if err != nil || ctx == nil {
		return openssl.SSLTLSEXTErrNoAck 
	}
	logger.Debugf("Changed context for: %s.", host)
	
	// set proper ctx
	ssl.SetSSLCtx(ctx)

	/*
	* SSL_set_SSL_CTX() only changes certs as of 1.0.0d
	* adjust other things we care about
	*/

	ssl.SetVerify(ctx.VerifyMode(), ctx.GetVerifyCallback())
	ssl.SetVerifyDepth(ctx.GetVerifyDepth())

	ssl.ClearOptions(ssl.GetOptions() &^ ctx.GetOptions())
	ssl.SetOptions(ctx.GetOptions())

	return openssl.SSLTLSExtErrOK 
}
```