RetryPolicy LWT support

[Lorak-mmk]

RetryPolicy should be able to behave differently when a query is LWT. For example, DefaultRetryPolicy should avoid retrying on next host in that case, so that it doesn't break LWT optimization.
To do this, we need to add isLWT field to QueryInfo struct, and adapt DefaultRetryPolicy to that.

Side notes:
QueryInfo should be marked non-exhaustive, so that we are able to add more fields to it in the future.
It could also implement Default, or have new() method, so that users are able to construct it. User may want to construct it to test their custom policies.

Maybe we should split QueryInfo into

• QueryInfo, passed to the policy in new_session (and perhaps retry? Not sure how the session are used), and containing fields that are bound to the query, like is_idempotent or isLWT.
• AttemptInfo containing info about specific attempt, passed to decide_should_retry. It would contain info about a specific attempt, like consistency or query error.

Why? To better utilize type system, documenting that query-related fields can't change between attempts.