[Feature] Retry network errors

[lubird]

The SDK should have a mechanism to cope with failed data uploads because of intermittent network errors or other issues. The retry strategy should be:

1. Retry network errors (socket timed out, etc.)
2. Retry server errors (HTTP 5xx)
3. Do NOT retry client errors (HTTP 4xx) except HTTP 429

[nd4p90x]

This can be marked as "won't fix", the current version supports the retry rules as stated in the issue. There is also a back-off feature with defaults that match current requirements.