Clarify limitations of callbacks
Because callbacks are run in the event loop thread, any blocking operation that would utilize the event loop will hang. For example, using session.prepare() or session.execute() would have this effect. I've attached a basic script to demonstrate the problem.
Unfortunately, the docs aren't clear about this, and a user will simply see the operation hang. At the least, we should improve the documentation. It might also be possible to perform some basic checks for this. For example, checking the thread ID and comparing against the event loop thread's ID in prepare() and execute() might work. However, I'm not sure how expensive that would be, and furthermore, it's difficult to communicate the error to the user. Exceptions that are raised in callabacks are logged and ignored, so the user would only see the exception if they had a try/except wrapper.