Don't allow initializing schema for Tx

[m110]

Similar to how watermill does it: https://github.com/ThreeDotsLabs/watermill-sql/blob/bc0272d92e9475871a52182dc341db4483e22dbb/pkg/sql/publisher.go#L67

If a transaction is passed to eventstore.SQL, it should return an error when initializing the schema. Otherwise, the transaction might be implicitly committed.