Dispose SslStream in ShutdownSocket

[bgrainger]

It doesn't appear that the SslStream allocated in MySqlSession.InitSslAsync is ever disposed. This might result in native memory leaks.

We probably need to dispose the X509Certificate2 allocated in that method, too.

[bgrainger]

RE: X509Certificate

Starting with the .NET Framework 4.6, this type implements the IDisposable interface. ... For apps that target the .NET Framework 4.5.2 and earlier versions, the X509Certificate class does not implement the IDisposable interface and therefore does not have a Dispose method.

Could start disposing it in builds that support net46 and/or netstandard2.0.

[bgrainger]

Fixed in 0.20.2.

[bgrainger]

Fixed X509Certificate2 in 0.23.0.