fix(retry): miscellaneous improvements to the default retry mechanisms

[kelvin-lu]

Summary

Props to @yuhao900914 (and I assume @dantetam in collaboration writing the Java SDK) for doing a critical reading of the logic in the retry mechanisms.

Fixes a couple of logical bugs that could result in errors down the line:

• Fixes an issue where the lastRetry flag is not computed correctly and can cause events to be retried more than necessary
• Fixes an issue where the wrong # of events are counted when removing due to 400 issues
• Makes an improvement to the logic checking of when tot stop retry loops

Checklist

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• Does your PR have a breaking change?:

[ajhorst]

@kelvin-lu is this PR still valid?