Option to use sessionStorage instead of localStorage

[acordiner]

When the retryQueue option is enabled in the Segment integration, it will queue events in localStorage. This means that unsent events may be left behind in localStorage and easily viewable by other people on a shared/public device. For privacy reasons, it would be great to be able to use sessionStorage instead of localStorage so that the unsent events are cleared when the session ends. Happy to contribute a PR if this sounds like a good idea!