Offline caching improvements - Delay between and backoff when 429 is on

The SDK guideline has a couple of new items regarding offline caching: https://github.com/getsentry/develop/pull/509

Retry on the background is tracked here: #803 but two main items are new:

1. If rate-limit was hit, make sure the caching transport is aware. Meaning: don't read all files from disk to send to the HTTP transport when that's basically dropping everything due to rate-limit
2. Add a delay when reading from disk to avoid bursts of events which could trigger rate limit

* https://github.com/getsentry/sentry-cocoa/issues/1671
* https://github.com/getsentry/sentry-java/issues/1926