Recycle MySqlDataReader instances

[bgrainger]

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.

[bgrainger]

The implementation of this in v2.3.0 breaks the following code (which is an unexpected pattern, but not "wrong"):

using var connection = ...
using var reader = ...
connection.Close();
// reader disposed afterwards

#1459 (comment)

It also seems to break Dapper's QueryAsync<>, which isn't using that exact pattern (AFAICT) but may trigger some code path that exposes a race condition in MySqlConnector.

The solution may be to make MySqlDataReader as slim as possible (maybe just referencing a ResultSet and MySqlCommand?), and pool and recycle the more heavy-weight ResultSet object.