retry: reduce total number of retries

Retries in restic try to solve two main problems:
- retry a temporarily failed operation
- tolerate temporary network interruptions

The first problem only requires a few retries, whereas the last one benefits
primarily from spreading the requests over a longer duration.

Increasing the default multiplier and the initial interval works for
both cases. The first few retries only take a few seconds, while later
retries quickly reach the maximum interval of one minute. This ensures
that the total number of retries issued by restic will remain at around
21 retries for a 15 minute period. As the concurrency in restic is
bounded, retries drastically reduce the number of requests sent to a
backend. This helps to prevent overloading the backend.

internal/backend/retry/backend_retry.go

@@ -108,6 +108,9 @@ func (be *Backend) retry(ctx context.Context, msg string, f func() error) error
 	bo := backoff.NewExponentialBackOff()
 	bo.MaxElapsedTime = be.MaxElapsedTime
 
+	bo.InitialInterval = 1 * time.Second
+	bo.Multiplier = 2
+
 	if fastRetries {
 		// speed up integration tests
 		bo.InitialInterval = 1 * time.Millisecond