4.x: Introduce exponential retry backoff policy

[dkropachev]

Make driver wait before retry. It is needed to mitigate retry storms that can happen in certain cases.

Fixes: https://github.com/scylladb/java-driver/issues/481

[dkropachev]

Documentation in multiple places is not adjusted to describe the new implementations. Often it refers to other existing classes or interfaces and does not list newly added params.

Is it necessary to introduce a new separate category of retry policies? I think from the user perspective it may be hard to understand how does the RetryPolicy interact with BackOffRetryPolicy. Ideally the new retry policy should be just an implementation of RetryPolicy interface.

It is definitely easier, I will try to make it part of regular retry