Split QueryInfo (used in RetryPolicy) into QueryInfo and AttemptInfo

[Lorak-mmk]

Originally raised in #1155

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.