Wait an appropriate delay before SyncWorker retries after soft errors (bitfireAT/davx5#337)

* Use appropriate delayUntil value for retrying syncs on 503s.

Crop server suggested retryAfter value to self defined min/max values and use a reasonable default value if non-existent.

* Add tests for getDelayUntil

* Wait appropriate delay, before retrying sync after a soft error happened

* Increase max and default sync delays after soft errors

* Increase initial backoff time for SyncWorker retries

* Minor getDelayUntil changes

* Minor changes

- store delayUntil in seconds
- pass duration instead of timestamp to Thread.sleep
- other minor changes

* Use Instant instead of Long timestamps

* Correct calculation of blocking duration

* Indicate soft error occurred on 503 server message

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Co-authored-by: Ricki Hirner <hirner@bitfire.at>