Since there can only be one active MySqlDataReader per MySqlConnection, we could create just one instance and recycle it when it's disposed.

This could allow a user to start using a disposed reader without failure (once it's been handed out to a different caller). That currently fails with an ObjectDisposedException, so no existing code would be broken, but people could start writing new, broken code.

Related: npgsql/npgsql#1649, #1264.