couchbase2: custom BestEffortRetryStrategy's exponential backoff cap is not respected

Suggested Release Note:

When using the `couchbase2` protocol with a custom `BestEffortRetryStrategy`, the cap on the exponential backoff was not respected, which could lead to undesirably long retry delays. 

`Backoff.java` only caps the timeout if a the retry context has a "previous delay time". For protostellar, the retry context never has this, since we create a new request wrapper for every retry attempt.

The fix is to update `Backoff.java` with the latest version from reactor-addons.