Check for multiple statements in prepare

Consider checking by default if someone does something like conn.prepare("<stmt 1>; <stmt 2>"). This will silently only run stmt 1, which is probably not what the user wants.

Right now, I believe we have some tail checking here, but I think it's only under extra-check feature. It also might be disabled then too. I think we should probably check this always, since I don't see a case where anybody wouldn't want this detected.