Prevent concurrent use of a single session id

[mattrobenolt]

Right now, we attempt to share the same connection, which results in an error.

This is a mistake by the user/driver/whatever and this behavior on PlanetScale becomes nondeterministic. PlanetScale ultimately serializes these parallel requests against the underlying mysqld in some capacity, but the order and behavior becomes nondeterministic.

In here, it makes sense to me, that instead of serializing these into our mysqld, we can use this opportunity to actually prevent it and return an error that more suggests that the behavior is wrong.

While this differs from what PlanetScale does, I think that's a useful deviation from our implementation especially if the intention is to use a tool like this for local dev or in CI environments, we'd like to surface this incorrect behavior as a useful error rather than still "allowing" it with our own nondeterministic behavior.