couchbase2: Needs to use exponential backoff

When it decides to bypass the default BestEffortRetryStrategy, RetryOrchestratorProtostellar is using a fixed backoff of 50 mills rather than exponential backoff. Instead, it should use the same backoff algorithm as BestEffortRetryStrategy.

Also, investigate whether it matters that ProtostellarBaseRequest.context() returns a context whose lastRetryDuration() method always returns null.