parseDSNParams manipulates global TLS config

[x6j8x]

Issue description

When connecting to multiple databases from the same program using TLS the current logic in parseDSNParams regarding the tlsconfig.ServerName is counterintuitive. It manipulates the global TLSConfig instance from the registry by setting the tlsconfig.ServerName property.

This results in an error like this (when trying to talk to multiple AWS Aurora databases):

x509: certificate is valid for cluster-cfcc3334-[..].eu-west-1.rds.amazonaws.com, cluster-cfcc3334-[..].eu-west-1.rds.amazonaws.com, not cluster-a865301d-[..].eu-west-1.rds.amazonaws.com

The easiest fix would be to take a copy of the global TLSConfig and use this copy for the DB specific parameter (like ServerName). Like this:

dsn.go / starting line 509

	if tlsConfig, ok := tlsConfigRegister[name]; ok {
		localTlsConfig := *tlsConfig
		if len(localTlsConfig.ServerName) == 0 && !localTlsConfig.InsecureSkipVerify {
			host, _, err := net.SplitHostPort(cfg.Addr)
			if err == nil {
				localTlsConfig.ServerName = host
			}
		}

		cfg.TLSConfig = name
		cfg.tls = &localTlsConfig
	} else {
		return errors.New("invalid value / unknown config name: " + name)
	}

Configuration

Driver version (or git SHA): 2e00b5c

Go version: 1.7.5

Server version: AWS Aurora 5.6.10a

[x6j8x]

duplicate to issue #536